Fixed column expression handling of the OmitAlias flag. Fixes bulk updat...

Dear Datanucleus Team,

Bulk updates were not working for me using PostgreSQL 9.1. I tracked the problem down to static handling of ColumnExpression generation, which causes the ColumnExpression class to ignore the omitTableFromString flag when called in the context of generating a bulk update SQL expression. The parameters for postgres correctly note that aliases in the "SET" clause are not allowed.

This pull request resolves the issue for me. I am using datanucleus 3.2.x, hence I put this on the 3.2.x branch. I would sincerely appreciate if you could generate a new 3.2.x release with this fix. Otherwise I hope this fix makes it into 3.3.x.

Regards,
Seth