generated excel files can not be opened by WPS

[retsyo]

all the generated excel files under .\libexcel\tests can not be opened by https://www.wps.com/office-free, but can be opened by LibreOffice though.

So there may be not an issue related to libexcel. But I note it down, so if someone meet same question then he can report too

[weimzh]

libexcel generates .xls file in older Excel 7.0 format, which WPS might not support.