For any request, am now adding following to the metadata for the InvocationRequest's request payload Map:
MethodType (GET, POST, etc)
Only time this will not be added is if is a POST request where an InvocationRequest is passed and is not binary content type (application/octet-stream).
Default request method type will be POST if not explicitly set otherwise in the metadata Map passed to the Client's invoke() method.
If doing a GET request, can just pass null value as the payload to the Client's invoke() method. Only problem is compiler will complain because makes method signature ambigous so can either cast the null to Object type, or just pass the url as payload (but will not be used for anything).