HTTP: RequestBuilder: blocking marshalling of RequestEntity

**Issue by [markusjura](https://github.com/markusjura)**
_Monday Nov 23, 2015 at 08:05 GMT_
_Originally opened as https://github.com/akka/akka/issues/18992_

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The [RequestBuilder](https://github.com/akka/akka/blob/releasing-akka-stream-and-http-experimental-2.0-M1/akka-http/src/main/scala/akka/http/scaladsl/client/RequestBuilding.scala#L47) can be used to create an `HttpRequest` object. One `apply` method uses `Await.result` to marshal the scala object to a `RequestEntity`. This means that this blocks the current thread on the execution context provided: https://github.com/akka/akka/blob/releasing-akka-stream-and-http-experimental-2.0-M1/akka-http/src/main/scala/akka/http/scaladsl/client/RequestBuilding.scala

Instead we could return a `Future[RequestEntity]` for the apply methods which pass in the `content` as `T` and not directly as `RequestEntity`.

Start in akka-http/src/main/scala/akka/http/scaladsl/client/RequestBuilding.scala, especially the RequestBuilder apply methods that accept content as T. Trace the Await.result marshalling path and assess the Future[RequestEntity] API proposed in the issue; done means those overloads no longer block the execution context and their callers remain coherent.

Written by the indexing model from the issue text.