Provide more flexible handling of the asType extension method (Scala only)

[rigolepe]

def asType: Future[T] = when(Status.Ok)

def when(status: Status): Future[T] =
  futureResponse.map { resp ⇒
    resp.body.getOrElse {
      val message =
        if (resp.status != status.code) s"The response has no typed body because the request was not successful (status = ${resp.status})."
        else "The response has no typed body despite status 200."
      throw new IllegalArgumentException(message)
    }
  }
}

extra: wrap all HTTP codes (e.g. 200 in Status.OK)