Http: client side connection abort leads to noisy warning in server

**Issue by [jrudolph](https://github.com/jrudolph)**
_Thursday Jul 16, 2015 at 13:59 GMT_
_Originally opened as https://github.com/akka/akka/issues/18015_

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Also, if the connection was really aborted attempting to send 500 response is useless.

```
Internal server error, sending 500 response (akka.stream.StreamTcpException)
```

/cc @sirthias

No files, tests, or entry points are named in the issue. Reproduce a client-side connection abort against the HTTP server, then verify that the server no longer emits the noisy warning or attempts to send a useless 500 response.

Written by the indexing model from the issue text.