HandleInvokeChaincode should have returned the pb.ChaincodeMessage from
the target handler as the response payload but it was returning the
payload from the response. The handler now returns the original message.

The TestChaincodeInvokeChaincode test has also been restored and is no
longer skipped by default.

Change-Id: I42aeb8504d70dd9435f55050cb7f0da3f71c8977
Signed-off-by: Matthew Sykes <sykesmat@us.ibm.com>