MDL-59539 oauth2/issuer: Changed baseurl for guessing image URL

The url, which was used for guessing the image, was queried with the
key 'discoveryurl', which is not defined in the context of an issuer.
The baseurl should be sufficient to retrieve the favicon.ico from a
server. That's why the key for was changed from 'discoveryurl' to
'baseurl'.

lib/classes/oauth2/api.php

@@ -349,7 +349,7 @@ public static function get_user_field_mappings(issuer $issuer) {
      */
     protected static function guess_image($issuer) {
         if (empty($issuer->get('image'))) {
-            $baseurl = parse_url($issuer->get('discoveryurl'));
+            $baseurl = parse_url($issuer->get('baseurl'));
             $imageurl = $baseurl['scheme'] . '://' . $baseurl['host'] . '/favicon.ico';
             $issuer->set('image', $imageurl);
             $issuer->update();