Convert the @font-feature-values declarations to lowercase

The @font-feature-values declaration identifiers are asci case insensitive.
We should convert all of them to lowercase to be able to reduce the same
declarations. Also gecko stores them as lowercase in gfxFontFeatureValueSet.

components/style/stylesheets/font_feature_values_rule.rs

@@ -166,7 +166,7 @@ impl<'a, 'b, 'i, T> DeclarationParser<'i> for FFVDeclarationsParser<'a, 'b, T>
                        -> Result<(), ParseError<'i>> {
         let value = input.parse_entirely(|i| T::parse(self.context, i))?;
         let new = FFVDeclaration {
-            name: Atom::from(&*name),
+            name: Atom::from(&*name).to_ascii_lowercase(),
             value: value,
         };
         update_or_push(&mut self.declarations, new);