hco Small akka-http-core changes as a preparation for the Java API

Here is a small set of cleanups and changes, I already had in the Java API branch but which apply generally and can be discussed independently.

The only possibly controversial change is the removal of the extra `HttpResponse` constructor. I found that in many cases it doesn't really help but instead confuses the overload resolution, i.e. constructing a `HttpResponse` with `HttpResponse(200, entity = "OK")` isn't possible any more with the extra constructor. IMO, if we want to allow `HttpResponse(200, "OK")` we should reorder the parameters of HttpResponse, instead, to allow for this kind of syntax.

/cc @sirthias
Any particular reason to use `immutable.Seq(...)` instead of `List(...)`?