Merge pull request #812 from paulholden/hook-403

Use original string hook callback definition for backwards compat.

db/hooks.php

@@ -27,7 +27,7 @@
 $callbacks = [
     [
         'hook' => \core\hook\output\before_http_headers::class,
-        'callback' => [auth_saml2\local\hooks\output\before_http_headers::class, 'callback'],
+        'callback' => 'auth_saml2\local\hooks\output\before_http_headers::callback',
         'priority' => 0,
     ],
 ];