(#320) - Parse XmlLang as an identifier, not a CSS string
CSS strings are quoted, like "foo". Our type for the xml:lang attribute was being parsed as a CSS string, so an attribute like
xml:lang="es"
would be parsed incorrectly, as the inner string would have had to be
xml:lang="'es'"
So, we parse this attribute as an identifier now.
Fixes #320 (closed)