set_quirks_mode was a no-op and the selector MatchingContext hardcoded
QuirksMode::NoQuirks, so class/id selectors were always case-sensitive even on
quirks-mode (no-doctype) pages where browsers match .foo / #bar
ASCII-case-insensitively. Store the parser-reported quirks mode on DomTree, pass
it into the selector context, and skip the exact-case #id index fast path in
quirks mode so it falls through to case-insensitive matching.
Fixes#499