Avoid setAttribute errors from invalid attributes, fixes #392

[gijsk]

The original markup that trips these exceptions is things that look like <table width="90% border="0> which gets you an attribute with name 0; passing that (or something with a "." or other invalid characters) to setAttribute can throw an exception, which we should catch.

[gijsk]

As noted in #392, I tried to use element.attributes.setNamedItem first, but I can't cloneNode attribute instances in jsdom ( jsdom/jsdom#2478 ) and so I couldn't figure out how to make that work. I think in general we don't care about these attributes anyway (only really about valid HTML attrs), so we might as well drop them on the floor when setting them doesn't work.