Support include_root_in_json for ActiveResource properly.

This commit is a backport from
https://github.com/rails/activeresource/pull/29. The
ActiveResource's include_root_in_json option is broken for 3.2.x.
Merge pull request #10578 from dingle/lu/back_port_to_json_fix

Support include_root_in_json for ActiveResource properly.