Commit nested transactions when exiting the context

[o-fedorov]

This change is required for PostgreSQL, I tested it with version 11.

Without a commit statement, nested transactions are silently rolled back, the behavior is quite surprising.

[o-fedorov]

Also, could not cover the change with tests, since the tests are written for SQLite only. Please tell me if you would like me to still add tests and perhaps provide CI configuration for PostgreSQL.

[mcfunley]

I added support for postgres in the test suite, and committed these changes here:
5c16c6a

And I added some test coverage for this here:
d43de3a

Thank you! Will cut a release shortly.