HttpExecutionContext should call delegate's prepare.

The HttpExecutionContext used the default implementation of prepare which just returns ```this```, ignoring any work that the delegate ExectutionContext may be doing. In my case I'm configuring Akka to use a custom MessageDispatcher which does some work during prepare which is not used when running Java code.

I wasn't sure the best way to test this; any suggestions for improvement are welcome.

I'd really appreciate it if this could be back ported to 2.2.x and 2.3.x if accepted.
You don't need this method, because the `HttpExecutionContext` constructor can be used to wrap the prepared delegate.