wymEqual tests against unparsed HTML by default.

This has been giving me extra work for some time now. For some inexplicable reason, IE7 keeps inventing new attributes every now and then, showing up in tests, making them fail.

This and the below change to `normalizeHtml` are a new approach; instead of a blacklist which IE7 seems to actively try to outsmart, we now have a whitelist.

After IE7 is not supported any more, we would probably not require either, because `attr.specified` would probably be enough.