Returning values from filters

[thekid]

For easier testing, if filter() returned whatever Invocation::proceed() returned (which is typically the case, most filters are implemented w/ return $invocation->proceed($request, $response);), and Invocation::proceed() returned whatever routing returns, then we could simplify the following:

// Before
private function filter(Request $request): Request {
  new BehindProxy()->filter(
    $request,
    new Response(new TestOutput()),
    new Invocation(Routing::cast(function($req, $res) use(&$passed) { $passed= $req; }))
  );
  return $passed;
}

// After
private function filter(Request $request): Request {
  return new BehindProxy()->filter(
    $request,
    new Response(new TestOutput()),
    new Invocation(Routing::cast(($req, $res) ==> $req))
  );
}

On a side note, Invocation could be more liberal in what it accepts, calling Routing::cast() internally.