MDL-41219 course: Make properties of course_modinfo read-only

added phpdocs and unittests

rss/file.php

@@ -79,11 +79,7 @@ if ($token==="$inttoken") {
     if ($course = $DB->get_record('course', array('id' => $courseid))) {
         $modinfo = get_fast_modinfo($course);
 
-        if (!isset($modinfo->instances[$componentname])) {
-            $modinfo->instances[$componentname] = array();
-        }
-
-        foreach ($modinfo->instances[$componentname] as $modinstanceid=>$cm) {
+        foreach ($modinfo->get_instances_of($componentname) as $modinstanceid=>$cm) {
             if ($modinstanceid==$instanceid) {
                 $context = context_module::instance($cm->id, IGNORE_MISSING);
                 break;