Function similar to servlet getParameterMap on request

When implementing some action proxyfing/forwarding request, it's useful to be able to get request parameters, from query string and body, like `HttpServletRequest.getParameterMap`.

```scala
      val queryString = req.queryString.foldLeft(
        Map[String, String]()) { (m, e) ⇒
          val (k, v) = e
          v.headOption.fold(m) { s ⇒ m + (k -> s) }
        }

      val post = (req.body match {
        case AnyContentAsFormUrlEncoded(params) ⇒ params
        case mp @ AnyContentAsMultipartFormData(_) ⇒
          mp.asFormUrlEncoded getOrElse Map[String, Seq[String]]()
        case _ ⇒ Map[String, Seq[String]]()
      }).foldLeft(Map[String, String]()) { (m, e) ⇒
        val (k, v) = e
        v.headOption.fold(m) { s ⇒ m + (k -> s) }
      }

      val params: : Map[String, String] = post ++ queryString
```

Also see http://stackoverflow.com/questions/13355442/how-to-get-all-request-parameters-in-play-and-scala
Empty instance