fix for dbcolumn not used in few places

ForeignKey with dbColumn specified does not generate proper sqldiff, this fix will generate the proper column name. 

Example problematic model is : 

```python
class TestSite(models.Model):
    SiteId = models.AutoField(primary_key=True)
    CreatedByUserId = models.ForeignKey("TestSite",db_column='ParentId')

    class Meta:
        db_table = "TestSite"
```