FIX: do not ignore directives which have no values, such as 'upgrade-…

…insecure-requests'

src/Models/Directive.php

@@ -398,7 +398,7 @@ public function getDirectiveValue(bool $useFakeNonce = false) : string
      */
     public function getDirectiveValueForPolicy(array $values) : string {
         if(count($values) == 0) {
-            return "";
+            return $this->Key . ";";
         } else {
             $values = array_unique($values);
             $directive = $this->Key . " " . implode(" ", $values) . ";";