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

foolip has just created a new issue for https://github.com/whatwg/dom:

== Make createAttribute() lowercase the input in HTML documents ==
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.

See https://github.com/whatwg/dom/issues/72