Convert tagName to uppercase ASCII per spec

http://dom.spec.whatwg.org/#dom-element-tagname
auto merge of #836 : bfrohs/servo/834upperTagName, r=jdm

http://dom.spec.whatwg.org/#dom-element-tagname

I added `window.alert(elem.tagName);` to line 14 of `test_bindings.js` to test (segfault at line 17), and it indeed uppercased the tagName. I didn't add that to this commit, however, because `tagName` is already tested on line 50 of that file. I'm assuming the duplication isn't necessary. Am I correct in that assumption? Or should tests be added somewhere for this? Where?