make void element test case insensitive

[Conduitry]

Fixes #336, but also fixes other issues where writing a void element in anything but lowercase would throw an error.

[Conduitry]

Eep I don't know what the heck these build failures are about

[Conduitry]

Hm. I don't know what a good way to handle this would be. The issue is the <Link> component used in a few tests. With this change, the parser now expects that to be a void element. Should components not be allowed to have the same names (case insensitively) as void elements? One other possibility would be to specifically exclude any elements that match a component we're declaring we're using - but at the stage where we're parsing the HTML, we probably don't know yet what the components are.

[Conduitry]

Running into some other weird case sensitivity stuff, making me think that more normalization is needed, rather than just changing this one regex. I'll open another issue.

[Conduitry]

IMO #341 is how this should all be addressed instead. But that still leaves open the question of how to handle components whose names are the same as void elements.

[Conduitry]

I'm going to close this PR without merging it. The important stuff has also been captured elsewhere, and the way forward is going to be a bit more complicated than this.