fixing pypdf table rendering issues due attribute sanitization

gluon.sanitizer.sanitize() is removing width and other need attributes to correctly render tables with pyfpdf

This change doesn't touch sanitize itself, it only add allowed attributes to gluon.generics.pyfpdf_from_html(html), so it should not pose any security issue, but please review if I'm missing something

See the related discussion in https://groups.google.com/d/msg/web2py/61hDnA7gQaY/gMUFl1r3yGQJ

Missing the test, sorry (but in the thread there are steps to reproduce and make a basic test)