db_change_field doesn't isn't right on PostgreSQL

Interesting that this never was picked up way earlier than now but the syntax for altering a column in PostgreSQL in wrong in the PostgreSQL driver :(

According to the 8.1 docs (http://www.postgresql.org/docs/8.3/static/sql-altertable.html) SET cannot be used to set the value of rows in a column. which is what the current driver attempts to do.

I think what happened was the driver was ported to handle the typecasting stuff but the actual content migration stuff failed until recently when a test was created to test it.

Attached is a patch that works. Test bots will pass it cause there are no postgreSQL test bots to test this :(

But I have test it and it does work.