= io: fix TcpConnection not handling intermediate empty writes in compound writes correctly
+ http: move `withDefaultHeaders` from HttpRequest up to HttpMessage
! routing: simplify and speed up CompletionMagnet
! httpx: remove MetaUnmarshallers.scala, fold only member into FormDataUnmarshallers.scala
+ httpx: add FromMessageUnmarshaller, FromRequestUnmarshaller and FromResponseUnmarshaller definitions

Implements #293 for the unmarshalling side
+ testkit: deprecate `entityAs[T]`, replace with `responseAs[T]`
+ httpx: extend MarshallingContext with support for specifying additional HTTP headers

Also: explicitly forbid using marshalers with RequestBuilding, which produce chunked requests
+ httpx: introduce `ToResponseMarshaller` and supporting infrastructure
! routing: upgrade to new ToResponseMarshaller, closes #293

Removes `RequestContext::marshallingContext`. Also: the `produce` directive looses its `status` and `headers` parameters.
Use the new `ToResponseMarshaller.fromMarshaller` creator instead.
Merge pull request #545 from spray/ticket/293

Close ticket #293, smaller improvements