Add support for ON DELETE SET DEFAULT

[makmn1]

Follow up to a discussion and pull requests #474 and #475

This pull request adds support for the SET DEFAULT action for ON DELETE on a foreign key constraint. It's similar to the current :nilify_all and {:nilify, columns} options, but for setting the column values to their default values. Additionally, database support is different between each of the three supported databases:

Database	SET DEFAULT – all columns	SET DEFAULT – partial columns
PostgreSQL	✔️	✔️
MySQL / MariaDB	✖️	✖️
SQL Server (MSSQL)	✔️	✖️

Sources for SET DEFAULT behavior

• PostgreSQL
• MySQL
• MSSQL

[greg-rychlewski]

@makmn1 Thanks very much for the PR. Would you be able to add default_all unit tests for TDS?

[makmn1]

Good catch, I added those missing tests and also updated the PG unit test for testing default_all with a composite key

[warmwaffles]

I didn't know this feature existed in postgres. I don't think it exists in sqlite. If this is added, I'll need to update the adapter to raise an error that is easy to understand.

[makmn1]

@warmwaffles It looks like it does exist in SQLite, but you can't choose columns (so similar to how MSSQL supports)

[greg-rychlewski]

Thanks for adding this :)