[WR] negotiate request body type

Related to [Pivotal Card 75990622](https://www.pivotaltracker.com/story/show/75990622), although obviously not the key focus.

Assigned @tie-rack.

In writing the online voter API we wrote tests for responding with a 415 Unsupported Media Type when the request body was an unsupported media type. However, Pedestal Toolbox doesn't actually have the capability to do such. This adds that capability within the same negotiate-content-type function.

One of the assumptions here is that you'd never want to specify different acceptable media types for the request body and the response body.
It makes sense that we should have an unsupported-media-type response. I just don't think we should have a flow through the interceptor that sets one response and then another. If one has precedence (which I suspect should be unsupported-media-type) then it should be set and the new context returned immediately.