PCBC-226 Use HTTP POST if "keys" are specified

Unfortunately we need to mix arguments on the URL and in
the payload of the HTTP POST due to a limitation inside
the CouchDB layer. It will silently ignore ALL other options
than the "keys" attribute in the payload :-(

Change-Id: I89c0c8f47f6b8fe5a602bd8d4eb26525698966c4
Reviewed-on: http://review.couchbase.org/28721
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>