Support show default value for virtual column

There's an edge case where a virtual column referencing a column with default value is created, the virtual column returns `nil` if the user doesn't force model reloading (`#reload`).

Test failing:
- test_fills_auto_populated_columns_on_creation

# Investation

As of now, I found out that the generated sql on insert doesn't include the virtual column, and it seems to be on purpose (See `PostgreSQL::Column#has_default?` or `Column#auto_populated?`). Hence the next step would be to verify the related behaviour with the postgresql adaptor directly to see when the value is added to the model.