WPDB: When a `db.php` drop-in is being used, and it doesn't explicitly define itself as connecting to MySQL, skip the character set checks. This ensures that existing drop-ins won't accidentally run checks that they don't support.

See #21212.


git-svn-id: https://develop.svn.wordpress.org/trunk@30375 602fd350-edb4-49c9-b593-d223f7449a82