Header verification

[mfeckie]

I've been looking through the codebase looking for a way to have the mocked API endpoints check request headers. I can't see an obvious way to do it. Is there an obvious way I'm missing or would it require some work to add such a feature?

For example, have the mocked endpoint respond if a content-type: application/json header was present, but fail if it was content-type: text/html or similar