Add expect_serialized_response option to RCP Client set up

Currently the RPC Client auto unserialize's the result from the server. This limits the use of the client to servers written in PHP that return a serialized result.

I wanted to remove this all together, but instead added I parameter to modify the behaviour, so not to break BC. I've added expect_serialized_response, which auto defaults to false for BC reasons. When set to true the unserialize is skipped when retrieving the results.

I've updated the existing tests to account for the default value.