=head2 update
- $obj->update;
+ $obj->update \%columns?;
Must be run on an object that is already in the database; issues an SQL
UPDATE query to commit any changes to the object to the database if
required.
+Also takes an options hashref of C<< column_name => value> pairs >> to update
+first. But be aware that this hashref might be edited in place, so dont rely on
+it being the same after a call to C<update>.
+
=cut
sub update {