Fixes mootools#2402 for 1.2. Overwrite String contains with the MooTo…

…ols version.

Firefox 18 implements .contains, and ES6 might do that as well. That breaks
some stuff, so we have to make sure it's overwritten with the 'correct'
MooTools version.

Source/Native/String.js

@@ -14,6 +14,10 @@ provides: String
 ...
 */
 
+try {
+	delete String.prototype.contains;
+} catch(e){}
+
 String.implement({
 
 	test: function(regex, params){