Revert #26826 and add documentation

This reverts commit a01cf703 as explained in the comment to #26826:

Realized that this PR caused the following warning in Travis CI:

```
/home/travis/build/rails/rails/activesupport/lib/active_support/dependencies.rb:293: warning: loading in progress, circular require considered harmful - /home/travis/build/rails/rails/activesupport/lib/active_support/core_ext/hash/indifferent_access.rb
```

Indeed, `active_support/core_ext/hash/indifferent_access.rb` **needs** to require `active_support/hash_with_indifferent_access.rb` in order to access the class `ActiveSupport::HashWithIndifferentAccess`.

The other way around, though, is not _strictly_ required, unless someone tries (like I did in the [gist above](https://gist.github.com/claudiob/43cc7fe77ff95951538af2825a71e5ec)) to use `ActiveSupport::HashWithIndifferentAccess` by only requiring `active_support/hash_with_indifferent_access.rb` without first requiring `active_support/core_ext/hash/indifferent_access.rb`.

I think the solution to this is to revert this PR and instead change the documentation to explicitly state that **developers should not require 'active_support/hash_with_indifferent_access'** if all they want is to use `ActiveSupport::HashWithIndifferentAccess` – instead they should require `active_support/core_ext/hash/indifferent_access.rb`.

1 files changed, 6 insertions, 1 deletions

diff --git a/activesupport/lib/active_support/hash_with_indifferent_access.rb b/activesupport/lib/active_support/hash_with_indifferent_access.rb
index a56943d99d..7ecc5c19bd 100644
--- a/activesupport/lib/active_support/hash_with_indifferent_access.rb
+++ b/activesupport/lib/active_support/hash_with_indifferent_access.rb
@@ -1,6 +1,5 @@
 require "active_support/core_ext/hash/keys"
 require "active_support/core_ext/hash/reverse_merge"
-require "active_support/core_ext/hash/indifferent_access"
 
 module ActiveSupport
   # Implements a hash where keys <tt>:foo</tt> and <tt>"foo"</tt> are considered
@@ -41,6 +40,12 @@ module ActiveSupport
   #   rgb = { black: '#000000', white: '#FFFFFF' }.with_indifferent_access
   #
   # which may be handy.
+  #
+  # To access this class outside of Rails, require the core extension with:
+  #
+  #   require "active_support/core_ext/hash/indifferent_access"
+  #
+  # which will, in turn, require this file.
   class HashWithIndifferentAccess < Hash
     # Returns +true+ so that <tt>Array#extract_options!</tt> finds members of
     # this class.