Unions not supported in subqueries

[muglug]

Found and fixed this bug in the PHP version:

This query

SELECT * FROM (SELECT * FROM `foo` UNION ALL SELECT * FROM `bar`) AS `baz`

is treated as

SELECT * FROM (SELECT * FROM `foo`) AS `baz`

Here's the commit with the fix in the PHP version, you should be able to port it relatively straightforwardly if you run into the same issue.