Add ability to preserve order in MySQL ON DUPLICATE KEY UPDATE.

Add ability to preserve order in MySQL ON DUPLICATE KEY UPDATE.

Added support for the parameters in an ON DUPLICATE KEY UPDATE statement on
MySQL to be ordered, since parameter order in a MySQL UPDATE clause is
significant, in a similar manner as that described at
:ref:`updates_order_parameters`.  Pull request courtesy Maxim Bublis.

Pull-request: https://github.com/zzzeek/sqlalchemy/pull/462
Change-Id: If508d8e26dbd3c55ab1e83cf573fb4021e9d091e