MDL-66968 php74: array_key_exists() for objects is deprecated

Replace it for correct property_exists() when the element
being inspected is a property of object/class.

Amended and squased changes:
- keep mongo unmodified. The information is array, hence correct.
- fix a couple of messaging phpdocs that were incorrect.

Amended take#2:
- As far as mongo resturns BSONDocument that is ArrayObject, aka
implements ArrayAccess, we have decided to explicitly cast results
to array so existing array_key_exists() and other accesses will
continue working the same.

lib/grade/grade_category.php

@@ -2192,7 +2192,7 @@ class grade_category extends grade_object {
         $children_array = array();
         foreach ($category->children as $sortorder=>$child) {
 
-            if (array_key_exists('itemtype', $child)) {
+            if (property_exists($child, 'itemtype')) {
                 $grade_item = new grade_item($child, false);
 
                 if (in_array($grade_item->itemtype, array('course', 'category'))) {