Enable `Performance/UnfreezeString` cop

In Ruby 2.3 or later, `String#+@` is available and `+@` is faster than `dup`.

```ruby
# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
end

Benchmark.ips do |x|
  x.report('+@') { +"" }
  x.report('dup') { "".dup }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]
Warming up --------------------------------------
                  +@   282.289k i/100ms
                 dup   187.638k i/100ms
Calculating -------------------------------------
                  +@      6.775M (± 3.6%) i/s -     33.875M in   5.006253s
                 dup      3.320M (± 2.2%) i/s -     16.700M in   5.032125s

Comparison:
                  +@:  6775299.3 i/s
                 dup:  3320400.7 i/s - 2.04x  slower

```

9 files changed, 9 insertions, 9 deletions

diff --git a/railties/lib/rails/commands/dbconsole/dbconsole_command.rb b/railties/lib/rails/commands/dbconsole/dbconsole_command.rb
index 806b7de6d6..0fac7d34a0 100644
--- a/railties/lib/rails/commands/dbconsole/dbconsole_command.rb
+++ b/railties/lib/rails/commands/dbconsole/dbconsole_command.rb
@@ -75,7 +75,7 @@ module Rails
         args += ["-P", "#{config['password']}"] if config["password"]
 
         if config["host"]
-          host_arg = "#{config['host']}".dup
+          host_arg = +"#{config['host']}"
           host_arg << ":#{config['port']}" if config["port"]
           args += ["-S", host_arg]
         end
diff --git a/railties/lib/rails/generators/generated_attribute.rb b/railties/lib/rails/generators/generated_attribute.rb
index f7fd30a5fb..3f20f5a718 100644
--- a/railties/lib/rails/generators/generated_attribute.rb
+++ b/railties/lib/rails/generators/generated_attribute.rb
@@ -153,7 +153,7 @@ module Rails
       end
 
       def inject_options
-        "".dup.tap { |s| options_for_migration.each { |k, v| s << ", #{k}: #{v.inspect}" } }
+        (+"").tap { |s| options_for_migration.each { |k, v| s << ", #{k}: #{v.inspect}" } }
       end
 
       def inject_index_options
diff --git a/railties/lib/rails/info.rb b/railties/lib/rails/info.rb
index d5c9973c6b..3df36efc4c 100644
--- a/railties/lib/rails/info.rb
+++ b/railties/lib/rails/info.rb
@@ -41,7 +41,7 @@ module Rails
       alias inspect to_s
 
       def to_html
-        "<table>".dup.tap do |table|
+        (+"<table>").tap do |table|
           properties.each do |(name, value)|
             table << %(<tr><td class="name">#{CGI.escapeHTML(name.to_s)}</td>)
             formatted_value = if value.kind_of?(Array)
diff --git a/railties/lib/rails/source_annotation_extractor.rb b/railties/lib/rails/source_annotation_extractor.rb
index 2d66a4dc7d..d7170e6282 100644
--- a/railties/lib/rails/source_annotation_extractor.rb
+++ b/railties/lib/rails/source_annotation_extractor.rb
@@ -50,7 +50,7 @@ module Rails
       # If +options+ has a flag <tt>:tag</tt> the tag is shown as in the example above.
       # Otherwise the string contains just line and text.
       def to_s(options = {})
-        s = "[#{line.to_s.rjust(options[:indent])}] ".dup
+        s = +"[#{line.to_s.rjust(options[:indent])}] "
         s << "[#{tag}] " if options[:tag]
         s << text
       end
diff --git a/railties/test/console_helpers.rb b/railties/test/console_helpers.rb
index 8350fce5ee..67f55fdc45 100644
--- a/railties/test/console_helpers.rb
+++ b/railties/test/console_helpers.rb
@@ -9,7 +9,7 @@ module ConsoleHelpers
   def assert_output(expected, io, timeout = 10)
     timeout = Time.now + timeout
 
-    output = "".dup
+    output = +""
     until output.include?(expected) || Time.now > timeout
       if IO.select([io], [], [], 0.1)
         output << io.read(1)
diff --git a/railties/test/generators/app_generator_test.rb b/railties/test/generators/app_generator_test.rb
index 14cdd8b5c9..1169633244 100644
--- a/railties/test/generators/app_generator_test.rb
+++ b/railties/test/generators/app_generator_test.rb
@@ -964,7 +964,7 @@ class AppGeneratorTest < Rails::Generators::TestCase
 
   def test_after_bundle_callback
     path = "http://example.org/rails_template"
-    template = %{ after_bundle { run 'echo ran after_bundle' } }.dup
+    template = +%{ after_bundle { run 'echo ran after_bundle' } }
     template.instance_eval "def read; self; end" # Make the string respond to read
 
     check_open = -> *args do
diff --git a/railties/test/generators/plugin_generator_test.rb b/railties/test/generators/plugin_generator_test.rb
index 20acd23329..468a9c9e89 100644
--- a/railties/test/generators/plugin_generator_test.rb
+++ b/railties/test/generators/plugin_generator_test.rb
@@ -737,7 +737,7 @@ class PluginGeneratorTest < Rails::Generators::TestCase
 
   def test_after_bundle_callback
     path = "http://example.org/rails_template"
-    template = %{ after_bundle { run "echo ran after_bundle" } }.dup
+    template = +%{ after_bundle { run "echo ran after_bundle" } }
     template.instance_eval "def read; self; end" # Make the string respond to read
 
     check_open = -> *args do
diff --git a/railties/test/generators/shared_generator_tests.rb b/railties/test/generators/shared_generator_tests.rb
index aa577e4234..5a1257e5c1 100644
--- a/railties/test/generators/shared_generator_tests.rb
+++ b/railties/test/generators/shared_generator_tests.rb
@@ -83,7 +83,7 @@ module SharedGeneratorTests
 
   def test_template_is_executed_when_supplied_an_https_path
     path = "https://gist.github.com/josevalim/103208/raw/"
-    template = %{ say "It works!" }.dup
+    template = +%{ say "It works!" }
     template.instance_eval "def read; self; end" # Make the string respond to read
 
     check_open = -> *args do
diff --git a/railties/test/isolation/abstract_unit.rb b/railties/test/isolation/abstract_unit.rb
index 96d21b3ae2..2a8c6d8f97 100644
--- a/railties/test/isolation/abstract_unit.rb
+++ b/railties/test/isolation/abstract_unit.rb
@@ -71,7 +71,7 @@ module TestHelpers
     end
 
     def extract_body(response)
-      "".dup.tap do |body|
+      (+"").tap do |body|
         response[2].each { |chunk| body << chunk }
       end
     end