Make $ownerKey nullable.

src/Ardent/Ardent.php

@@ -448,7 +448,7 @@ public function belongsTo($related, $foreignKey = NULL, $otherKey = NULL, $relat
 	 * @param  string  $id
 	 * @return \Illuminate\Database\Eloquent\Relations\BelongsTo
 	 */
-	public function morphTo($name = null, $type = null, $id = null, $ownerKey) {
+	public function morphTo($name = null, $type = null, $id = null, $ownerKey = null) {
 		// If no name is provided, we will use the backtrace to get the function name
 		// since that is most likely the name of the polymorphic interface. We can
 		// use that to get both the class and foreign key that will be utilized.