Investigate strange response in HttpServerSpec

**Issue by [johanandren](https://github.com/johanandren)**
_Thursday Apr 21, 2016 at 08:11 GMT_
_Originally opened as https://github.com/akka/akka/issues/20377_

---

In #20300 a new method `shutdownBlueprint` was introduced to make sure we shutdown the server test fixture after test so that `assertAllStagesStopped` would make us notice resource leaks.

To make it pass for tests that send a request, assert some properties of the request, never produce a response, close the incoming stream (network bytes) there is a `HTT` produced on the output stream of bytes before it shuts down.

May not have any relevance for actual cases as the test is doing weird things, but would be good if someone understood why and was sure it's not a problem.

Start in HttpServerSpec and trace the test fixture shutdown introduced by shutdownBlueprint in #20300. Reproduce the request that asserts properties, produces no response, closes the incoming stream, and then runs assertAllStagesStopped; inspect the resulting HTT bytes. Done means the response is explained and its impact is confirmed as safe or actionable.

Written by the indexing model from the issue text.