Update InteractsWithPivotTable.php

As with #22484 , when inserting a pivot record via `->attach()` the `InteractsWithPivotTable.php` trait doesn't check to see if the pivot table has a model representation. It' simply finds the table name and inserts a new record. Because of this, the timestamps are passed as `Carbon` instances and then turn to strings via its `toString()` method. If the consumer is using a pivot Model, it should use that models dateFormat to format by, not the `toString()` method. This simply news-up the pivot model, fetches it's dateformat and formats the `$fresh` date against it.

src/Illuminate/Database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php

@@ -313,6 +313,11 @@ protected function baseAttachRecord($id, $timed)
     protected function addTimestampsToAttachment(array $record, $exists = false)
     {
         $fresh = $this->parent->freshTimestamp();
+
+        if ($this->using) {
+            $pivotModel = new $this->using;
+            $fresh = $fresh->format($pivotModel::getDateFormat());
+        }
 
         if (! $exists && $this->hasPivotColumn($this->createdAt())) {
             $record[$this->createdAt()] = $fresh;