fix pivot timestamp columns without parent

- check to see if `pivotParent` exists before getting its timestamp columns.
- if there is no parent, fallback to the parent `Model` methods.
- add tests. 1 for when a parent exists, and 1 for when it doesn't

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

@@ -214,7 +214,7 @@ public function hasTimestampAttributes()
      */
     public function getCreatedAtColumn()
     {
-        return $this->pivotParent->getCreatedAtColumn();
+        return ($this->pivotParent) ? $this->pivotParent->getCreatedAtColumn() : parent::getCreatedAtColumn();
     }
 
     /**
@@ -224,7 +224,7 @@ public function getCreatedAtColumn()
      */
     public function getUpdatedAtColumn()
     {
-        return $this->pivotParent->getUpdatedAtColumn();
+        return ($this->pivotParent) ? $this->pivotParent->getUpdatedAtColumn() : parent::getUpdatedAtColumn();
     }
 
     /**

tests/Database/DatabaseEloquentPivotTest.php

@@ -4,6 +4,7 @@
 
 use Mockery as m;
 use PHPUnit\Framework\TestCase;
+use Illuminate\Database\Eloquent\Model;
 use Illuminate\Database\Eloquent\Relations\Pivot;
 
 class DatabaseEloquentPivotTest extends TestCase
@@ -119,6 +120,29 @@ public function testPivotModelTableNameIsSingular()
 
         $this->assertEquals('pivot', $pivot->getTable());
     }
+
+    public function testPivotModelWithParentReturnsParentsTimestampColumns()
+    {
+        $parent = m::mock('Illuminate\Database\Eloquent\Model');
+        $parent->shouldReceive('getCreatedAtColumn')->andReturn('parent_created_at');
+        $parent->shouldReceive('getUpdatedAtColumn')->andReturn('parent_updated_at');
+
+        $pivotWithParent = new Pivot();
+        $pivotWithParent->pivotParent = $parent;
+
+        $this->assertEquals('parent_created_at', $pivotWithParent->getCreatedAtColumn());
+        $this->assertEquals('parent_updated_at', $pivotWithParent->getUpdatedAtColumn());
+    }
+
+    public function testPivotModelWithoutParentReturnsModelTimestampColumns()
+    {
+        $model = new DummyModel();
+
+        $pivotWithoutParent = new Pivot();
+
+        $this->assertEquals($model->getCreatedAtColumn(), $pivotWithoutParent->getCreatedAtColumn());
+        $this->assertEquals($model->getUpdatedAtColumn(), $pivotWithoutParent->getUpdatedAtColumn());
+    }
 }
 
 class DatabaseEloquentPivotTestDateStub extends \Illuminate\Database\Eloquent\Relations\Pivot
@@ -152,3 +176,7 @@ class DatabaseEloquentPivotTestJsonCastStub extends \Illuminate\Database\Eloquen
         'foo' => 'json',
     ];
 }
+
+class DummyModel extends Model
+{
+}