Allow customize response that is generated in akka http when a request timeout is exceed

**Issue by [matheuslima](https://github.com/matheuslima)**
_Saturday Oct 08, 2016 at 16:27 GMT_
_Originally opened as https://github.com/akka/akka/issues/21638_

---

Today, the response generated looks like:

```
HttpResponse(StatusCodes.ServiceUnavailable, entity = "The server was not able " +
"to produce a timely response to your request.\r\nPlease try again in a short while!")
```

I need add some headers to all responses generated from my server, but the directive respondWithHeaders seems not work with timed out responses, due the fact the default response is injected by akka http server.

Start by tracing where Akka HTTP generates the default response for request timeouts and compare that path with the respondWithHeaders directive. Confirm how timeout responses bypass the directive, then define done as allowing the generated response to be customized so required headers can be included.

Written by the indexing model from the issue text.