check headers

Сheck the headers in the request

Although the API Blueprint allows multiple responses for a request, Drakov needs a matching pair in order to resolve which response to send back. That's the price we have to pay to have deterministic behaviour.

If you want Drakov to respond with a HTTP 401 for a given request, you should specify it. In this example a HTTP 401 will be sent back when the `Authorization` is `Basic foo`.

Eg.

```
Check Authorization header

+ Request

    + Headers

        Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

+ Response 200

### Delete the thing with bad auth header value [DELETE]
+ Request

    + Headers

        Authorization: Basic foo
        
+ Response 401
```