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.

admin/repository.php

@@ -123,13 +123,13 @@ if (($action == 'edit') || ($action == 'new')) {
             }
             $instanceoptionnames = repository::static_function($repository, 'get_instance_option_names');
             if (!empty($instanceoptionnames)) {
-                if (array_key_exists('enablecourseinstances', $fromform)) {
+                if (property_exists($fromform, 'enablecourseinstances')) {
                     $settings['enablecourseinstances'] = $fromform->enablecourseinstances;
                 }
                 else {
                     $settings['enablecourseinstances'] = 0;
                 }
-                if (array_key_exists('enableuserinstances', $fromform)) {
+                if (property_exists($fromform, 'enableuserinstances')) {
                     $settings['enableuserinstances'] = $fromform->enableuserinstances;
                 }
                 else {