Improve String#strip_heredoc

Saves about 6 MB, about 40% faster.

**strip_heredoc.rb**

```ruby
require "active_support/core_ext/object/try"
require "get_process_mem"

class String
  def strip_heredoc
    indent = scan(/^[ \t]*(?=\S)/).min.try(:size) || 0
    gsub(/^[ \t]{#{indent}}/, '')
  end
end

if ENV["MEASURE_MEMORY"] == "yes"
  mem = GetProcessMem.new
  GC.start
  GC.disable
  10000.times do
    <<-MSG.strip_heredoc
      xhr and xml_http_request methods are deprecated in favor of
      `get :index, xhr: true` and `post :create, xhr: true`
    MSG
  end
  before = mem.mb

  after = mem.mb
  GC.enable
  puts "Before: #{before} MiB"
  puts "After: #{after} MiB"
  puts "Diff: #{after - before} MiB"
end
```

**patched_strip_heredoc.rb**

```ruby
require "active_support/core_ext/object/try"
require "get_process_mem"

class String
  def patched_strip_heredoc
    gsub(/^#{scan(/^[ \t]*(?=\S)/).min}/, "".freeze)
  end
end

if ENV["MEASURE_MEMORY"] == "yes"
  mem = GetProcessMem.new
  GC.start
  GC.disable
  10000.times do
    <<-MSG.patched_strip_heredoc
      xhr and xml_http_request methods are deprecated in favor of
      `get :index, xhr: true` and `post :create, xhr: true`
    MSG
  end
  before = mem.mb

  after = mem.mb
  GC.enable
  puts "Before: #{before} MiB"
  puts "After: #{after} MiB"
  puts "Diff: #{after - before} MiB"
end
```

**Before**

```
$ MEASURE_MEMORY=yes ruby strip_heredoc.rb
Before: 44.73828125 MiB
After: 44.7734375 MiB
Diff: 0.03515625 MiB
```

**After**

```
$ MEASURE_MEMORY=yes ruby patched_strip_heredoc.rb
Before: 37.9765625 MiB
After: 38.015625 MiB
Diff: 0.0390625 MiB
```

`44.7734375 -  38.015625 = 6.75`

=> **Saves about 6.75 MiB**

**benchmark.rb**

```ruby
require "benchmark/ips"
require_relative "./strip_heredoc"
require_relative "./patched_strip_heredoc"

def original
  <<-MSG.strip_heredoc
    xhr and xml_http_request methods are deprecated in favor of
    `get :index, xhr: true` and `post :create, xhr: true`
  MSG
end

def patched
  <<-MSG.patched_strip_heredoc
    xhr and xml_http_request methods are deprecated in favor of
    `get :index, xhr: true` and `post :create, xhr: true`
  MSG
end

Benchmark.ips do |x|
  x.report("original") { original }
  x.report(" patched") { patched  }
  x.compare!
end
```

```
$ ruby -v benchmark.rb
ruby 2.2.2p95 (2015-04-13 revision 50295) [x86_64-darwin14]
Calculating -------------------------------------
            original     5.652k i/100ms
             patched     6.477k i/100ms
-------------------------------------------------
            original     54.076k (± 5.7%) i/s -    271.296k
             patched     74.557k (± 6.2%) i/s -    375.666k

Comparison:
             patched:    74557.0 i/s
            original:    54076.4 i/s - 1.38x slower
```

=> **About 38% faster**

1. Clone rails project `git clone git@github.com:rails/rails.git`
2. Apply this patch to
`activesupport/lib/active_support/core_ext/string/strip.rb`
3. `cd activesupport`
4. run `rake`
5. And tests passed:

```
➜ activesupport $ rake
/Users/Juan/.rubies/ruby-2.2.2/bin/ruby -w -I"lib:test"
"/Users/Juan/.rubies/ruby-2.2.2/lib/ruby/2.2.0/rake/rake_test_loader.rb"
 "test/**/*_test.rb"

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......................................................................S.
SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS

Finished in 15.343004s, 214.2344 runs/s, 24902.4898 assertions/s.

3287 runs, 382079 assertions, 0 failures, 0 errors, 48 skips

You have skipped tests. Run with --verbose for details.
```

1 files changed, 1 insertions, 2 deletions

diff --git a/activesupport/lib/active_support/core_ext/string/strip.rb b/activesupport/lib/active_support/core_ext/string/strip.rb
index 9fdd9d8d2e..a523e76b69 100644
--- a/activesupport/lib/active_support/core_ext/string/strip.rb
+++ b/activesupport/lib/active_support/core_ext/string/strip.rb
@@ -20,7 +20,6 @@ class String
   # Technically, it looks for the least indented non-empty line
   # in the whole string, and removes that amount of leading whitespace.
   def strip_heredoc
-    indent = scan(/^[ \t]*(?=\S)/).min.try(:size) || 0
-    gsub(/^[ \t]{#{indent}}/, '')
+    gsub(/^#{scan(/^[ \t]*(?=\S)/).min}/, ''.freeze)
   end
 end