Header.Get/Set re-canonicalize and allocate whenever the passed key
isn't already in canonical MIME header form. Sec-WebSocket-Key,
WWW-Authenticate, and content-type all miss the fast path; switch to
their canonical forms (Sec-Websocket-Key, Www-Authenticate,
Content-Type).
Enable the canonicalheader linter to catch future regressions,
excluded in test files since those already use non-canonical casing
in several places without a perf-sensitive path behind them.
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