Add support for setting the Last-Modified header easily

[thekid]

Example of how this simplifies calling code:

-use web\Headers;

 $items= $this->repository->newest(20);
 return View::named('blog')
-  ->header('Last-Modified', Headers::date($items[0]['date'] ?? null))
+  ->modified($items[0]['date'] ?? null)
   ->with(['items' => $items])
 ;

Example would be to use this as part of https://developer.mozilla.org/en-US/docs/Web/HTTP/Conditional_requests

[thekid]

Released in https://github.com/xp-forge/frontend/releases/tag/v6.4.0