deal with empty results, add 'displayed'

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WebDriverBase.php

@@ -82,7 +82,12 @@ protected function curl($http_method,
     }
     curl_close($curl);
 
-    return array('value' => $results['value'], 'info' => $info);
+    $value = null;
+    if (is_array($results) && array_key_exists('value', $results)) {
+      $value = $results['value'];
+    }
+
+    return array('value' => $value, 'info' => $info);
   }
 
   public function __call($name, $arguments) {

WebDriverElement.php

@@ -31,6 +31,7 @@ protected function methods() {
       'location_in_view' => 'GET',
       'size' => 'GET',
       'css' => 'GET',
+      'displayed' => 'GET',
     );
   }