bugfix: Fix headers not being sent for RunnableResponse

The callable must be called from the sendContent() method, rather than send(). Otherwise, sendHeaders() is never called.

lib/SimpleSAML/HTTP/RunnableResponse.php

@@ -65,7 +65,7 @@ class RunnableResponse extends Response
      *
      * @return self
      */
-    public function send(): object
+    public function sendContent(): object
     {
         return call_user_func_array($this->callable, $this->arguments);
     }