Make sure consumer stream request response is a valid consumer

Change-Id: Ibb8be9d4a5cf6d8990fa1beeb9733feb33a6a0ab
Reviewed-on: http://review.couchbase.org/31583
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>