Support show default value for virtual column

[BuonOmo]

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.