Introducing UnitTests for XmlFormatters to confirm that headers are always written before writing the body.

Issue https://github.com/aspnet/Mvc/issues/607.
Sorna, did we do anything ourselves in MVC to throw this kind of error?...i ask this because this check of headers being set after the response sent out should probably be in another layer (ex: httpabstractions)..scenario is that a user writing a middleware might do a similar thing and we should be throwing this kind of error then too...