expose internal HttpRequest rendering API to users

**Issue by [aappddeevv](https://github.com/aappddeevv)**
_Tuesday Dec 01, 2015 at 15:32 GMT_
_Originally opened as https://github.com/akka/akka/issues/19062_

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There are needs to render HTTPRequest objects to text independent of sending them through a connection. The HttpRequest rendering API could be exposed. The key class to expose is RequestRendererFactory. This may require exposing the RequestRenderingContext.

Here's the link to the google-group post describing this: [google-group](https://groups.google.com/forum/#!msg/akka-user/X374C-s8B-I/e-r6E37PAwAJ).

Another approach is to allow an EntityBody to be composed of HttpRequests and have the rendering happen e.g. expose the rendererer as a Marshaller somehow since the rendering can be represented as a flow inside a Marshaller.

Start by reading the internal RequestRendererFactory and RequestRenderingContext APIs mentioned in the issue, then review the linked Google Group discussion and the 11-comment thread for a decided direction. Done means users can render HttpRequest objects independently of a connection through a clearly exposed API, or through the selected EntityBody/Marshaller approach.

Written by the indexing model from the issue text.