Fix bug #630635: Print page size is incorrect: https://bugs.launchpad.net/inkscape/+bug/630635
Don't rotate the page if using a custom page size, because it confuses simpler printers which don't understand landscape mode, like laser cutters.
(Note: comments #3-7 in that bug report are a different issue, which affects printers which don't understand custom page sizes; this issue was fixed in commit eaa134e8)