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/message.php

@@ -77,7 +77,7 @@ if (($form = data_submitted()) && confirm_sesskey()) {
                     // Record the site preference.
                     $preferences[$processor->name.'_provider_'.$componentprovidersetting] = $value;
                 }
-            } else if (array_key_exists($componentprovidersetting, $form)) {
+            } else if (property_exists($form, $componentprovidersetting)) {
                 // We must be processing loggedin or loggedoff checkboxes. Store
                 // defained comma-separated processors as setting value.
                 // Using array_filter eliminates elements set to 0 above.