This commit adds a middleware function that checks any POST endpoint is
called with Content-Type: application/json. Returns `HTTP 415
Unsupported Media Type` if this is not the case.
TODO: this commit does not check that the payload is actually
well-formed JSON. If the payload is *not* valid JSON, muutaja will fail,
which will result in an `HTTP 500 Server error`.
This initial version is purposely extremily limited:
* `GET /ahoy` endpoint, just to check the server is alive
* `GET /items` endpoint, always returns the same item
* `POST items` endpoint, checks and print request payload, no
persistance
* calling any other route defaults to a 404
