Allow null value for route name in PRG plugin

[juriansluiman]

In a lot of PRG patterns the POST request happens to the same url as the GET request. This PR keeps the flexibility to set the route name but it also defaults to the currently matched route name.

Previously I did:

$route = $this->getEvent()->getRouteMatch()->getMatchedRouteName();

if (($result = $this->prg($route)) instanceof Response) {
    return $result;
}  elseif (false !== $result) {
    // Do stuff
}

Where now you're able to:

if (($result = $this->prg()) instanceof Response) {
    return $result;
}  elseif (false !== $result) {
    // Do stuff
}

Test is made to assert the behaviour.