Update 3_2_release_notes.md

Fixed typo and applied proper code highlighting.
Missing destroy command
Fix typos: the indefinite articles(a -> an)
Merge pull request #12216 from suginoy/a-an

Fix typos: the indefinite articles(a -> an).
Merge pull request #12215 from harshadsabne/master

Update 3_2_release_notes.md
Custom flash should be defined only for the class that defines it and it's subclasses.
Merge pull request #12222 from rcillo/fix-custom-flash

Custom flash should be defined only for the class that defines it and it's subclasses

Fixes #12057
Don't require using application_name before options

Before this commit options for `rails new` and `rails plugin new` had to
be passed in a strict order, trying to execute a following command:

    rails new -J path/to/app

resulted in an error.

This commit fixes the situation and allows to pass path to app anywhere
after `new`
Fixing comment typo in ActionController::Base
Merge pull request #12223 from adomokos/fixing_typo

Fixing comment typo in ActionController::Base
Removing ActiveSupport::Concern, it's not needed
Merge pull request #12225 from adomokos/no_need_for_active_support_concern

Removing ActiveSupport::Concern, it's not needed
Deprecated CircularReferenceError in AS::JSON::Encoding

As discussed with @jeremy on campfire, this is considered programmer
error and protecting against these kind of errors is out of scope for
the JSON encoder. A StackError will be raised instead, which is
aliased as AS::JSON::Encoding::CircularReferenceError with a
deprecation warning for Rails 4.1.

Removing this responsibility allowed me to remove a lot of complexity
from the encoder while still passing all existing tests.

This reuses ideas and code from @sergiocampama's #11728.

Also, "fixes" #11460.
Added coverage for bugs fixed in the previous commit
Added #jsonify in AS JSON Encoder
Replaced encode_json with ::JSON.generate
Added the option `ActiveSupport.default_json_encoder`

This option allows you to swap out the default JSON encoder used by
ActiveSupport::JSON.encode and Object#to_json. This allows you to
write custom JSON encoders to provide additional functionailities
(such as encode_json support and caching), or to use alternative JSON
libaries. The default is ActiveSupport::JSON::Encoding::JSONGemEncoder
which uses the built-in JSON gem.
Changelogs
No need to require json here, if json gem is still not loaded at this point then all bets are off
Raise an ArgumentError when options is not a Hash

This would help to catch errors such as JSON gem trying to call
to_json with a state_object:

  ArgumentError: Expected a Hash for options, got '#<JSON::Ext::
  Generator::State:0x007f8d7ea43400>' instead. This is likely a bug in
  your JSON encoder (ActiveSupport::JSON::Encoding::JSONGemEncoder).

  ArgumentError: Expected a Hash for options, got '#<JSON::Ext::Generator::State:0x007f97881fb0b8>' instead. This is likely a bug in your JSON encoder (ActiveSupport::JSON::Encoding::JSONGemEncoder).