Fixed aggreate aliasing not working properly

Hey again, previously

```coffee
select('t1', function (q) { q.agg({'foobar': 'count'}, q.p('id')) })
```

Would compile to

```sql
SELECT ([object Object](t1.id)) AS foobar FROM t1
```

This fix makes it compile to

```sql
SELECT count(t1.id) AS foobar FROM t1
```