Invoking an RPC with no expend input should not fail when content type is set to application/json

When I invoke an RPC which expects no input, it shouldn't matter what the content/type header is set to. Unfortunately you MUST have a content type other than application/json (or xml, or their variants) in order to get the call routed to the correct REST method. This happens because the code that deserializes the string into a composite node asserts that there be something defined.

Instead of enforcing content at the deserialization (which is not aware of whether input is required or not) we should allow deserialization to return null, and perform the proper validation when have the RPC call definition.