We can't test driver.New methods because of circular imports.
That's probably why the initial code had a map[string]interface{} as
registry: it removes a dependency import.
I prefer the remove the registry and have a registry returning a real
Driver. It will ease the development later.
The MySQL driver is using `CREATE TABLE IF NOT EXISTS`, which causes
the MySQL database to raise warnings. The golang driver collects
the warnings and return a composite object (mysql.MySQLWarnings) as
an error that needs to be properly handled.
This change stops the driver from failing in case there are only
warnings.
