Fix error in rescue rpcapi that prevents sending objects

The change restores the rescue_instance rpc's ability to send
instance objects from version 3.11 onwards, that was inadvertently
broken by https://review.openstack.org/#/c/62314

Unit test modified to check that rpc version passes instance object
instead of dict.

Change-Id: Icffea82da623d3aab4661554b33bd5959bfe2bd8
Closes-Bug: #1327145
Merge "Fix error in rescue rpcapi that prevents sending objects"