ADOdb pgsql fix: AlterColumnSQL conversion from int to bool

When changing type of a column from integer to boolean, PostgreSQL does
not provide implicit type cast, so the library needs to specify it via
USING statement when generating the SQL·

Reverse problem of 3fc6200386a975a204f6951acba4067b40ecabd0

This issue exists in ADOdb <= 5.17. Upstream bug report
http://phplens.com/lens/lensforum/msgs.php?id=19204