knop_database oncreate -keyfield's column cannot be numeric

This code will fail.

``` lasso
var(db) = knop_database(
    -database='database',
    -table='table',
    -maxrecords=$maxrecords,
    -keyfield='id',
    )
```

```
An unhandled failure during a web request
Error Code: 21
Error Msg: Is a directory
Error Stack: 
169:20 file.lasso
```

Use a varchar for `-keyfield`.
