fix bug with update existing pivot and polymorphic many to many

Author: taylorotwell

src/Illuminate/Database/Eloquent/Relations/MorphToMany.php

@@ -115,6 +115,19 @@ public function getRelationExistenceQuery(Builder $query, Builder $parentQuery,
         );
     }
 
+    /**
+     * Get the pivot models that are currently attached.
+     *
+     * @return \Illuminate\Support\Collection
+     */
+    protected function getCurrentlyAttachedPivots()
+    {
+        return parent::getCurrentlyAttachedPivots()->map(function ($record) {
+              return $record->setMorphType($this->morphType)
+                            ->setMorphClass($this->morphClass);
+        });
+    }
+
     /**
      * Create a new query builder for the pivot table.
      *