[dom] Make createAttribute() lowercase the input in HTML documents (#72)

The context for this can be found in https://bugzilla.mozilla.org/show_bug.cgi?id=1176313 and the bugs mentioned therein.

In summary, this looks like the best option for getting the case handling of attribute names in a somewhat consistent state, by lowercasing input where needed, and it avoids case-insensitive lookups.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/72