Use override keyword more prominently in Http codebase

**Issue by [ktoso](https://github.com/ktoso)**
_Wednesday Mar 09, 2016 at 15:08 GMT_
_Originally opened as https://github.com/akka/akka/issues/19986_

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In general the HTTP codebase does not use the override keyword too much.
AFAIR we had some discussions and were moving towards using it more, anyway most important is to be consistent so this should be applied over the entire codebase.

One instance of such places here: https://github.com/akka/akka/pull/19984

Start by reviewing the Akka HTTP codebase and the referenced pull request at https://github.com/akka/akka/pull/19984 to see the intended use of override. Identify the remaining inconsistent locations and verify that override usage is consistent across the HTTP codebase.

Written by the indexing model from the issue text.