add test for the implicit corotine decoration of XMLRPC methods

[graingert]

this is https://github.com/mosquito/aiohttp-xmlrpc/pull/31 without the controversial replacement for asyncio.coroutine

[graingert]

I hope to make a PR to remove this behaviour (for a future v1 release?) and require that all rpc_ methods return awaitables

[mosquito]

Please rebase