as_json support

to_json in ActiveSupport now looks for `as_json` which should convert the object into a json compatible format. Without this Rails 3.x will ignore the to_json methods in georuby and try to do it's best guess at a json format based on attributes.

Assuming anyone is using this already with Rails 3.x, this will change their json output. For the better, but it is still potentially a breaking change.
Isn't this a problem? Calling `#to_json` twice?