Formatting, capitalization, and punctuation fixes [ci skip]

Second try on this commit.
Merge pull request #13167 from fluxusfrequency/patch-3

Formatting, capitalization, and punctuation fixes [ci skip]
mention Active Record enums as major feature for 4.1. [ci skip]
remove nil in NullType#ref.

Return Nil is implicit in a method and this syntax is used in the others
classes
Merge pull request #13170 from acapilleri/remove_return_nil

remove nil in NullType#ref.
removed duplicate test case
Merge pull request #13171 from kuldeepaggarwal/test-case-updation

removed duplicate test case
Added failing test for json_escape striping quotation marks

Expanded test coverage for html_escape and json_escape
Fixed a long-standing bug in `json_escape` that strips quotation marks
Use lower case letters in unicodes sequences to match the new encoder's output
Added \u2028 \u2029 to json_escape
Also move html_esacpe regex to a constant (see 9d25af60)
Merge pull request #13073 from chancancode/json_escape

Fixed json_escape (again)
Review json_escape docs [ci skip]
update guides to new version of rails 3.2.x[ci skip]
Merge pull request #13177 from kuldeepaggarwal/doc-upgrade

update guides to new version of rails 3.2.x [ci skip]
Update guides welcome page to point to version 3.2.16 [ci skip]
Update changelog [ci skip]
Run Travis tests using Ruby 2.1.0-preview2 too
Retain ActionPack dependency on ActionView