add MethodNotAllowedException::getAllowedMethods()

[azjezz]

this PR adds the getAllowedMethods() method to the MethodNotAllowedException exception class.

this helps developers to send the correct Allow header value.

see : https://tools.ietf.org/html/rfc7231#section-7.4.1
see : https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Allow
see : https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/405

Note: this PR contains a small BC-break, in case someone is extending the MethodNotAllowedException class ( which is unlikely ).

Example :

try {
  list($handler, $data) = $router->routeRequest($request);

  $response = await $handler->handle(
    $request->withAttribute('data', $data)
  );

} catch(MethodNotAllowedException $e) {
  $response = new Response(405)
	|> $$->withAddedHeader('Allow', $e->getAllowedMethods());

} catch(NotFoundException $e) {
  $response = new Response(404);

}

await $emitter->emit($response); 
exit(0);

[azjezz]

this PR also fixes the build by updating dependencies ( hhast 4.0.1 uses static function variables )

[fredemmott]

Sorry for the delay - inlien comments and this also needs a rebase, but generally seems good.

[jjergus]

Thanks, this looks good to me!

Just one comment in the code + can you please address the lint warning?

[fredemmott]

Will merge once lint's fixed

the whole HEAD <-> GET reroute* should probably be removed in the future.

#15