Allow the use of BodyParsers in sub actions.

I found that it doesn't work to use a body parser in a sub action:

```scala
RequiresAuthentication(parse.json)("FacebookClient") { p =>
    Action(parse.json) {
      …
    }
  }
```

I don't think this is ideal (why do I have to pass parse.json to RequiresAuthentication now? :) ) but it is a start - what do you think?

The one thing that worries me is the cast in getRedirectionUrl
```scala
request.asInstanceOf[Request[AnyContent]]
```

I tried adjusting ScalaWebContext.java to take Request<A> but I didn't have any luck with that :(