sql_like_expression() doesn't handle underscores correctly

Having an underscore in the sql_like_expression() fails. The underscore ends up being double escaped (in MySQL, not sure about other DBMs).

It is escaped in the sql_like_expression() function itself. Then it gets escaped again by the sql_escape() function.

For example,

$db->sql_like_expression('foo_' . $db->get_any_char())

Results in a bad MYSQL error:

LIKE 'FOO\\_%'

To work it must be:

LIKE 'FOO\_%'