Don't serialize identity column if it's nil

Sending the NULL value in identity column when inserting a record would cause `Sequel::NotNullConstraintViolation` exception. This patches the `Coercer#to_record` to serialize the identity column only when it's assigned.

Let me know if you think it would be a good idea to update `repository_test.rb` with Postgresql integration as well. Maybe tests for specific RDBMSs could be turned on or off with environment variables (just like AR does it).