Revert "Add 'extra' method on XMLRPCRequest to pass along additional object."

This reverts commit 68d88169fde1d2ea68fd6987a2f196f483251432.
Revert "Revert "Add 'extra' method on XMLRPCRequest to pass along additional object.""

This reverts commit f647b35134cc24ac480408ce2755246a6bbcec66.