2 more quick JSON-related patches

Just some quick cleanup and documentation.

Remove obsolete ActiveSupport::JSON::Variable - http://dev.rubyonrails.org/ticket/9969
Add documentation for Hash#to_json and Enumerable#to_json -
http://dev.rubyonrails.org/ticket/9970

Please review :slight_smile: Thanks!

Cheers,
Chu Yeow