Add some tests for how content types and HTTP "Accept" header handling behave.

Hopefully this doesn't duplicate existing tests...

These are some tests that I wrote to characterize how Pyramid HTTP "Accept" header handling works while I was working on a bug in [Cornice](https://github.com/mozilla-services/cornice).

I'm in the Pyramid PyCon sprint room today so if you want to chat about these or tweak them, we can talk.