[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.
Yeah, that's a bug. We're fixing it now.