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

Same fix as be97535b for 3.1. Makes the method signature
more consistent with other DataExtension methods,
and comply with its subclass implementation in
Hierarchy->validate(). See accbd7f1e2 for more comments.

model/DataExtension.php

@@ -58,7 +58,7 @@ abstract class DataExtension extends Extension {
 	 * @param $validationResult Local validation result
 	 * @throws ValidationException
 	 */
-	public function validate(ValidationResult &$validationResult) {
+	public function validate(ValidationResult $validationResult) {
 	}
 	
 	/**