Preserve case of SVG/MathML foreign-content attributes - #284

minify-html lowercases every HTML attribute name, which breaks SVG (and would break MathML) foreign content: per the HTML spec, attribute names inside those subtrees are case-sensitive, so <svg viewBox="0 0 24 24"> renders differently from viewbox. Browsers and conformant parsers (e.g. html5lib) preserve the case; minify-html does not.

Repro:

minify("<svg viewBox=\"0 0 24 24\" preserveAspectRatio=\"xMidYMid\"></svg>")
=> <svg viewbox="0 0 24 24" preserveaspectratio=xMidYMid></svg>   (broken)

Changes:

• Track the SVG namespace during parsing (an svg element enters it, case-insensitive match) and preserve element + attribute name case inside it; HTML keeps lowercasing.
• Same fix in both implementations (minify-html and minify-html-onepass), since they share the test data.
• Closing-tag matching is now case-insensitive so </linearGradient> still matches <linearGradient>.
• Updated the shared test data: viewBox is expected preserved; an uppercase D on <path> is now treated as a distinct attribute (no lowercase-induced value removal).

Tests: cargo test -p minify-html -p minify-html-common -p minify-html-onepass — all green.