Prevents MySQL driver to failing when version table alredy exists

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.

driver/mysql/mysql.go

@@ -50,9 +50,12 @@ func (driver *Driver) Close() error {
 }
 
 func (driver *Driver) ensureVersionTableExists() error {
-	if _, err := driver.db.Exec("CREATE TABLE IF NOT EXISTS " + tableName + " (version int not null primary key);"); err != nil {
+	_, err := driver.db.Exec("CREATE TABLE IF NOT EXISTS " + tableName + " (version int not null primary key);")
+
+	if _, isWarn := err.(mysql.MySQLWarnings); err != nil && !isWarn {
 		return err
 	}
+
 	return nil
 }