Clarify parsing / handling of `authority-form` of request target

Nobody has claimed this yet.

**Issue by [jrudolph](https://github.com/jrudolph)**
_Monday Oct 05, 2015 at 11:45 GMT_
_Originally opened as https://github.com/akka/akka/issues/18643_

---

As detected in https://github.com/akka/akka/pull/18642#issuecomment-145501475 it seems that akka-http doesn't parse request targets correctly if the target is given in [`authority-form`](http://tools.ietf.org/html/rfc7230#section-5.3.3) which is only valid for `CONNECT` requests (that aren't supported by akka-http) anyway.

In an [akka-user post](https://groups.google.com/d/topic/akka-user/eAgsGyUzMu4/discussion), the error message was

```
akka.http.scaladsl.model.IllegalUriException: 'Host' header value of request to `openoox.com:8080` doesn't match request target authority: Host header: Some(Host: openoox.com:8080) request target authority:
```

which translates to

```
uri.toString == "openoox.com:8080"
hostHeader.toString == "Some(Host: openoox.com:8080)"
uri.authority == ""
```

So, this means the request probably was a CONNECT request and looked like this:

```
CONNECT openoox.com:8080 HTTP/1.1
```

In this case the uri is parsed as

```
uri.scheme == openoox.com
uri.path == 8080
```

which doesn't make any sense.

akka-http should either properly parse this into a model representing the authority form or outright reject requests like this.

Start by tracing akka-http’s request-target parsing for the CONNECT example and compare it with the RFC 7230 authority-form behavior described here. The issue names no files or tests; done means the implementation consistently represents or rejects authority-form targets instead of parsing `openoox.com:8080` as a scheme and path, with coverage for the chosen behavior.

Written by the indexing model from the issue text.