fix($http): ignore xhr.responseType setter exception if value is "json"

WebKit added support for the json responseType value on 09/03/2013 
https://bugs.webkit.org/show_bug.cgi?id=73648. Versions of Safari prior to 7
are known to throw when setting the value "json" as the response type. Other
older browsers implementing the responseType. Other browsers with infrequent
update cycles may also be affected.

The json responseType value can be ignored if not supported, because JSON
payloads are parsed on the client-side regardless.

Closes #6115