InsertSelect onConflict doesn't compile

while fixing #50 i found out that `onConflict` for InsertSelect actually doesn't compile with:
```type mismatch;
 found   : scalasql.core.Expr[Int]
 required: scalasql.query.Column[_]
```
reproduce:
```scala
Buyer.insert.select(
  identity,
  Buyer.select.filter(_.id === 1)
).onConflictIgnore(_.id)
```