make MathML attribute values case insensitive

[fred-wang]

MathML attributes are case-sensitive while HTML5 attributes are
case-insensitive. case-sensitiveness is probably not a problem for users
and it's easier for the parsing. However, WebKit developers writing or
reviewing patches have often considered doing case-insensitive
comparisons as that's consistent with the rest of the code base.

cc @rwlbuis

original report: https://lists.w3.org/Archives/Public/www-math/2016Aug/0000.html

[rwlbuis]

Firefox and WebKit both treat named colors in a case-insensitive way.
Firefox and WebKit both treat named math-variant in a case-sensitive way.

[fred-wang]

Resolution:

• https://lists.w3.org/Archives/Public/public-mathml4/2019May/0006.html
• agree to make case insensitive