Correctly wrapping a request message in a ResourceContainer.

Fixes #107.

@kdeus Care to give it a look?

#106 has a working sample app and the method `erpmcs.salesorders.update` will give the warning before the change but not after.
I'm sorry, that was always supposed to be `proto_message`. Fail by me.

I'll fix this, rebase, `git push -f` and ping you.