fix bug group by extend
Thank you!
I fixed bug group by extend
- Examples:
users table have columns: id and username
Tweet table have contain user_id column. user_id is foreign key of users table

```sql
   query = (User
                 .select(User, fn.COUNT(Tweet.id).alias('tweet_count'))
                 .join(Tweet, JOIN.LEFT_OUTER)
                 .group_by_extend(User.id).group_by_extend(User.username))
```
 
Then I run this query.

- Before I fix bug then result of query :
```sql
SELECT t1.id, t1.username, count(t2.id) as tweet_count
FROM users as t1
LEFT OUTER JOIN tweet as t2 on (t2.user_id = t1.id)
```

-  After I fix bug then result of query:
```sql
SELECT t1.id, t1.username, count(t2.id) as tweet_count
FROM users as t1
LEFT OUTER JOIN tweet as t2 on (t2.user_id = t1.id)
GROUP BY t1.id, t1.username  # I fixed to add group by
```