Remove variable for attribute, document why the change was made.

metatag.inc

@@ -130,16 +130,23 @@ class BackdropDefaultMetaTag implements BackdropMetaTagInterface {
         }
 
         // Combine the base configuration for this meta tag with the value.
-        $attribute = 'name';
         $element = $base_element + array(
           '#tag' => 'meta',
           '#attributes' => array(
-            $attribute => $this->info['name'],
             'content' => $value,
           ),
           '#weight' => $this->getWeight(),
         );
 
+        if (in_array($this->info['group'], array('facebook', 'open-graph'))) {
+          // Open Graph tags need a property attribute, instead of name.
+          $element['#attributes']['property'] = $this->info['name'];
+        }
+        else {
+          // Fall back to name attribute for all others.
+          $element['#attributes']['name'] = $this->info['name'];
+        }
+
         // Add header information if desired.
         if (!empty($this->info['header'])) {
           $element['#attached']['backdrop_add_http_header'][] = array($this->info['header'], $value);