Migrations: default_ not working for timestamp

I have a migration defined with a field
```haskell
field "created" timestamp notNull (defaultTo_ currentTimestamp_)
```
But the default value is not added to the column when running the migration.
If I add the default value manually, the migration doesn't remove the default value. Seems like it is just being ignored.

So when running an insert using `default_` in the timestamp position we get
```
"null value in column \"created\" violates not-null constraint"
```