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
Closes #6122