Do not return null if parameter is not correctly encoded

Considering the following route
```
GET /foo/:baz controllers.Foo.bar(baz)
```

``` scala
object Foo extends Controller {
  def bar(baz: String) = Action {
     Ok(JSON.stringify(baz))
  }
}
```

Now, let request the controller with content like:
```
$ curl http://lolcathost:9000/foo/bar:baz
```


In actual play-master, ''baz'' is not correctly encoded. ''baz'' parameter in ''bar'' action gets null.

In play <= 2.1.1, the string was passed "as-this" and not encoded in any way. This MAY break user's code, as it may receive a null parameter. This will lead to unexpected behaviour if request is miss-formated.


In this pull request I try to fallback to a safer behaviour and not to pass a null parameter to user code.