Optionally disable ON DUPLICATE KEY UPDATE

For MySQL, it would be nice if the ON DUPLICATE KEY UPDATE option could be disabled completely, by passing `:on_duplicate_key_update => false`. 

Unfortunately I have no idea how to write a test for this small modification that uses your conventions; I have verified that it doesn't break any existing tests (for mysql2), and that it indeed drops the ON DUP... clause from the INSERT statement.

Many thanks for the great gem!