Attribute selectors should compare names according to spec

[Ms2ger]

The HTML spec states

When comparing the name part of a CSS attribute selector to the names of namespace-less attributes on HTML elements in HTML documents, the name part of the CSS attribute selector must first be converted to ASCII lowercase. The same selector when compared to other attributes must be compared according to its original case. In both cases, the comparison is case-sensitive.

In particular, in a HTML document, [FOO] should

• match an HTML element with a namespace-less foo attribute
• not match an HTML element with a namespace-less FOO attribute
• not match a non-HTML element with a namespace-less foo attribute
• match a non-HTML element with a namespace-less FOO attribute
• not match an HTML element with a namespaced foo attribute
• match an HTML element with a namespaced FOO attribute
• not match a non-HTML element with a namespaced foo attribute
• match a non-HTML element with a namespaced FOO attribute
  (and we'll want tests for all those).

cc @SimonSapin

[SimonSapin]

I believe we’re doing this right already. [FOO] is equivalent to [|FOO], and so should not match any namespaced attribute.

For test cases: the HTML parser normalizes attribute names to ASCII lower case, so a FOO attribute can only be created through DOM manipulation.