XML-RPC: Add default values to IXR_Message for PHP 7.2 compatibility to avoid PHP Warnings.

Fixes #43216 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42652


git-svn-id: http://core.svn.wordpress.org/trunk@42481 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/IXR/class-IXR-message.php

@@ -9,12 +9,12 @@
  */
 class IXR_Message
 {
-    var $message;
-    var $messageType;  // methodCall / methodResponse / fault
-    var $faultCode;
-    var $faultString;
-    var $methodName;
-    var $params;
+    var $message     = false;
+    var $messageType = false;  // methodCall / methodResponse / fault
+    var $faultCode   = false;
+    var $faultString = false;
+    var $methodName  = '';
+    var $params      = array();
 
     // Current variable stacks
     var $_arraystructs = array();   // The stack used to keep track of the current array/struct