Allow current timestamp default to be specified for DateTimeTz type.

Currently there is no way to specify a current timestamp as a default when using the `DateTimeTz` type, as it will always be quoted by the system. For example, the generated schema would have:

`DEFAULT 'NOW()'`

Instead of the correct:

`DEFAULT NOW()`

This simple fix allows `DateTimeTz` to behave just as `DateTime` in this regard.