Add dataType override in ajaxOptions()

This change allows ajaxOptions to set jQuery dataType based on a value set on the adapter, defaulting to 'json' if not present. This allows rest_adapter to be extended and specify an alternate, like 'jsonp', for an third party API, without needing to override the entire ajaxOptions function.

I'd be happy to write a test for this, but I'm a little lost as to where to test for this type of change. The current Test Suite continues to pass.