Use a deep extend in wp.Uploader to ensure that the multipart_params object (and others) are cloned instead of referenced. fixes #20532.

git-svn-id: http://svn.automattic.com/wordpress/trunk@20577 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/plupload/wp-plupload.dev.js

@@ -25,7 +25,8 @@ if ( typeof wp === 'undefined' )
 			},
 			key;
 
-		this.plupload = $.extend( { multipart_params: {} }, wpPluploadDefaults );
+		// Use deep extend to ensure that multipart_params and other objects are cloned.
+		this.plupload = $.extend( true, { multipart_params: {} }, wpPluploadDefaults );
 		this.container = document.body; // Set default container.
 
 		// Extend the instance with options