Allow to use ‘as’ attribute on HTML ‘link’ element

‘as’ attribute can be used as additional hint for the browser when
preloading resources. This patch allows to use the ‘as’ attribute by
HeadLink helper.

Author: gszy

src/Helper/HeadLink.php

@@ -51,7 +51,8 @@ class HeadLink extends Placeholder\Container\AbstractStandalone
         'extras',
         'itemprop',
         'crossorigin',
-        'integrity'
+        'integrity',
+        'as'
     ];
 
     /**