Tighten consistency requirements for resource types

[kjetilk]

With the sound of the deadline of the milestone approaching with rising pitch, I decided to draft what I think is coming out of #301. There doesn't seem to be significant opposition to most of this given the test results, with the possible exception of the status code. I put 409 in anyway. We can conclude that debate in #301.

The PR should address both my initial concern, and @langsamu's first concern of changing resource types. The text isn't very extensive, and does not cover it in as much detail as we did on the issue, but I think we can enforce this by encoding the status codes in tests.