Fixed bug where using server_onupdate=<FetchedValue|DefaultClause>

Fixed bug where using server_onupdate=<FetchedValue|DefaultClause>
without passing the "for_update=True" flag would apply the default
object to the server_default, blowing away whatever was there.
The explicit for_update=True argument shouldn't be needed with this usage
(especially since the documentation shows an example without it being
used) so it is now arranged internally using a copy of the given default
object, if the flag isn't set to what corresponds to that argument.
[ticket:2631]