Bug Fix: InsertOrUpdateGeneratorPostgres

When updating a basic two-column many-to-many relationship table the statement generated by the InsertOrUpdateGeneratorPostgres incorrectly uses the SELECT keyword which causes in the pl/pgsql statement to fail.

This change uses the PERFORM keyword instead to ensure that a result is not returned.

Also, the statement was not being terminated with a semi-colon which was also an issue.