Add backticks for ON DUPLICATE statement in MySQL adapter

[mberlanda]

A database I am working on gave me the 1064 MYSQL error due to the usage of some restricted keywords:

https://dev.mysql.com/doc/refman/8.0/en/keywords.html

When tested in the client:

-- original generated query
INSERT INTO `users` (`id`, `admin`)
VALUES (123, 0)
ON DUPLICATE KEY UPDATE admin = VALUES(admin);

-- corrected working query
INSERT INTO `users` (`id`, `admin`)
VALUES (123, 0)
ON DUPLICATE KEY UPDATE `admin` = VALUES(`admin`);

[jamis]

Thank you!