fix incompatibility with newer Spreadsheet::ParseExcel (fixes #22)

in https://rt.cpan.org/Public/Bug/Display.html?id=93065,
Spreadsheet::ParseExcel decided to start treating 'auto' colors as black
instead of white, but this isn't correct for fill colors, so we have to
correct for that
note the XML::Twig bug (fixes #20)
changelog
Return missing comments, clean ws
Merge remote-tracking branch 'upstream/master'