Call super copyWithZone: in subclasses.

By not calling the superclass' implementation, we end up not actually
copying over all the properties from the superclass. In this case, if we
call -copy on an instance of an AFJSONRequestSerializer, the headers for
the serializer are not copied over (as well as anything else that was
changed in the AFHTTPRequestSerializer implementation).
Merge pull request #2062 from streeter/copywithzone-should-call-super

Call super copyWithZone: in subclasses.