Py_INCREF on return value from HttpResult_success

Change-Id: I1a3e35616a7ea57d3382b307621cbeb7591eef42
Reviewed-on: http://review.couchbase.org/26623
Reviewed-by: Mordechai Nunberg <mnunberg@haskalah.org>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>