[html5 spec] Update attribute serialization to escape `<` and `>`

[flavorjones]

Although originally (pre-2008) these characters were escaped during serialization, that was changed in 2008. Now the spec has changed back again, as detailed in this issue:

whatwg/html#6235

Notably it now looks like all the major browsers are backing this change, as noted in the replies to the only issue that has been opened documentating breakage.

This issue has been opened so that we update how Nokogiri serializes HTML5 documents to escape these characters.

Note that this spec change does NOT affect parsing; it only affects serialization.

cc @stevecheckoway for awareness