Don't type cast the default on the column

If we want to have type decorators mess with the attribute, but not the
column, we need to stop type casting on the column. Where possible, we
changed the tests to test the value of `column_defaults`, which is
public API. `Column#default` is not.
Less operations to do:

```ruby
default = schema_default(column) if column.has_default?
spec[:default] = default if default
```