Pass user_ctx down to fabric_rpc
Pass user_ctx down to fabric_rpc

The Options array in fabric_view_all_docs contains the {user_ctx, _}
tuple but we omitted to pass it down to the open_doc command. This
caused several incorrect behaviours for the _users database which has
some special properties for privacy reasons.

COUCHDB-3232

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb