Restore http callback signature

Corrected rest request handler to reflect previous behaviour of
abstracting specific rest results.

Change-Id: I4ba52288b3f434a6c526f339465a47f76ac6d972
Reviewed-on: http://review.couchbase.org/24028
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>