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

```

6 files changed, 7 insertions, 7 deletions

diff --git a/activesupport/lib/active_support/xml_mini/jdom.rb b/activesupport/lib/active_support/xml_mini/jdom.rb
index 7f94a64016..59c65db2d5 100644
--- a/activesupport/lib/active_support/xml_mini/jdom.rb
+++ b/activesupport/lib/active_support/xml_mini/jdom.rb
@@ -169,7 +169,7 @@ module ActiveSupport
       # element::
       #   XML element to be checked.
       def empty_content?(element)
-        text = "".dup
+        text = +""
         child_nodes = element.child_nodes
         (0...child_nodes.length).each do |i|
           item = child_nodes.item(i)
diff --git a/activesupport/lib/active_support/xml_mini/libxml.rb b/activesupport/lib/active_support/xml_mini/libxml.rb
index 0b000fea60..2a16932f03 100644
--- a/activesupport/lib/active_support/xml_mini/libxml.rb
+++ b/activesupport/lib/active_support/xml_mini/libxml.rb
@@ -55,7 +55,7 @@ module LibXML #:nodoc:
           if c.element?
             c.to_hash(node_hash)
           elsif c.text? || c.cdata?
-            node_hash[CONTENT_ROOT] ||= "".dup
+            node_hash[CONTENT_ROOT] ||= +""
             node_hash[CONTENT_ROOT] << c.content
           end
         end
diff --git a/activesupport/lib/active_support/xml_mini/libxmlsax.rb b/activesupport/lib/active_support/xml_mini/libxmlsax.rb
index dcf16e6084..a22a2c9cb7 100644
--- a/activesupport/lib/active_support/xml_mini/libxmlsax.rb
+++ b/activesupport/lib/active_support/xml_mini/libxmlsax.rb
@@ -23,7 +23,7 @@ module ActiveSupport
       end
 
       def on_start_document
-        @hash = { CONTENT_KEY => "".dup }
+        @hash = { CONTENT_KEY => +"" }
         @hash_stack = [@hash]
       end
 
@@ -33,7 +33,7 @@ module ActiveSupport
       end
 
       def on_start_element(name, attrs = {})
-        new_hash = { CONTENT_KEY => "".dup }.merge!(attrs)
+        new_hash = { CONTENT_KEY => +"" }.merge!(attrs)
         new_hash[HASH_SIZE_KEY] = new_hash.size + 1
 
         case current_hash[name]
diff --git a/activesupport/lib/active_support/xml_mini/nokogiri.rb b/activesupport/lib/active_support/xml_mini/nokogiri.rb
index 5ee6fc8159..4762a759d6 100644
--- a/activesupport/lib/active_support/xml_mini/nokogiri.rb
+++ b/activesupport/lib/active_support/xml_mini/nokogiri.rb
@@ -59,7 +59,7 @@ module ActiveSupport
             if c.element?
               c.to_hash(node_hash)
             elsif c.text? || c.cdata?
-              node_hash[CONTENT_ROOT] ||= "".dup
+              node_hash[CONTENT_ROOT] ||= +""
               node_hash[CONTENT_ROOT] << c.content
             end
           end
diff --git a/activesupport/lib/active_support/xml_mini/nokogirisax.rb b/activesupport/lib/active_support/xml_mini/nokogirisax.rb
index b01ed00a14..0bbb4e258a 100644
--- a/activesupport/lib/active_support/xml_mini/nokogirisax.rb
+++ b/activesupport/lib/active_support/xml_mini/nokogirisax.rb
@@ -39,7 +39,7 @@ module ActiveSupport
       end
 
       def start_element(name, attrs = [])
-        new_hash = { CONTENT_KEY => "".dup }.merge!(Hash[attrs])
+        new_hash = { CONTENT_KEY => +"" }.merge!(Hash[attrs])
         new_hash[HASH_SIZE_KEY] = new_hash.size + 1
 
         case current_hash[name]
diff --git a/activesupport/lib/active_support/xml_mini/rexml.rb b/activesupport/lib/active_support/xml_mini/rexml.rb
index 32458d5b0d..55a155d4ee 100644
--- a/activesupport/lib/active_support/xml_mini/rexml.rb
+++ b/activesupport/lib/active_support/xml_mini/rexml.rb
@@ -76,7 +76,7 @@ module ActiveSupport
           hash
         else
           # must use value to prevent double-escaping
-          texts = "".dup
+          texts = +""
           element.texts.each { |t| texts << t.value }
           merge!(hash, CONTENT_KEY, texts)
         end