ensure ON UPDATE test is case insensitive

ensure ON UPDATE test is case insensitive

Fixed regression caused by the DEFAULT rendering changes in 2.0.40
:ticket:`12425` where using lowercase `on update` in a MySQL server default
would incorrectly apply parenthesis, leading to errors when MySQL
interpreted the rendered DDL.  Pull request courtesy Alexander Ruehe.

Fixes: #12488
Closes: #12489
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12489
Pull-request-sha: b9008f747d21bc06a4006c99a47fc6aa99407636

Change-Id: If5281c52415e4ddb6c2f8aee191d2335f6673b35