Removed explicit pass-by-ref in DataExtension->validate()

Same fix as be97535 for 3.1. Makes the method signature
more consistent with other DataExtension methods,
and comply with its subclass implementation in
Hierarchy->validate(). See accbd7f for more comments.

model/DataExtension.php

@@ -58,7 +58,7 @@ public static function unload_extra_statics($class, $extension) {
 	 * @param $validationResult Local validation result
 	 * @throws ValidationException
 	 */
-	public function validate(ValidationResult &$validationResult) {
+	public function validate(ValidationResult $validationResult) {
 	}
 
 	/**