Fix IN replacement in virtual fields for MYSQL.

Adding a basic test case for 
https://github.com/cakephp/cakephp/pull/1449

Without the change, the 

    (SELECT id FROM comments WHERE id = (SELECT 1)) AS  `Comment__extra`'

would be (wrongly) modified to

    (SELECT id FROM comments WHERE id IN (SELECT 1)) AS  `Comment__extra`'