Enforce content type on barbican REST API

Before barbican moved from Falcon to Pecan, content types were coerced to application/json. That meant that a user could use a tool such as curl, omit the content type, and the call would succeed.

After moving to Pecan, the default content-type changed to application/x-www-form-urlencoded. This meant that a curl request without a content type specified would end up coming into barbican as a urlencoded string and would fail during JSONifying.

This blueprint proposes that we check and enforce the content-type coming in and return an http 415 error if it is not correct. That means that a curl request will have to ensure that it adds the content-type=application/json otherwise it will fail with an http 415 error.

Implementation of this bluepring will require:

- updating the barbican code to detect incorrect content types and return http 415
- updating the barbican documentation to describe this behavior and how to remedy
- updating the barbican tests to validate the behavior