caddyhttp: use canonical header key casing to avoid re-canonicalization (#7911)

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.

.golangci.yml

@@ -19,6 +19,7 @@ linters:
     - asciicheck
     - bidichk
     - bodyclose
+    - canonicalheader
     - decorder
     - dogsled
     - dupl
@@ -97,6 +98,9 @@ linters:
       - linters:
           - errcheck
         path: _test\.go
+      - linters:
+          - canonicalheader
+        path: _test\.go
     paths:
       - third_party$
       - builtin$

modules/caddyhttp/caddyauth/basicauth.go

@@ -212,7 +212,7 @@ func (hba HTTPBasicAuth) promptForCredentials(w http.ResponseWriter, err error)
 	if realm == "" {
 		realm = "restricted"
 	}
-	w.Header().Set("WWW-Authenticate", fmt.Sprintf(`Basic realm="%s"`, realm))
+	w.Header().Set("Www-Authenticate", fmt.Sprintf(`Basic realm="%s"`, realm))
 	return User{}, false, err
 }
 

modules/caddyhttp/encode/encode.go

@@ -499,10 +499,10 @@ func hasVaryValue(hdr http.Header, target string) bool {
 // http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html.
 func AcceptedEncodings(r *http.Request, preferredOrder []string) []string {
 	acceptEncHeader := r.Header.Get("Accept-Encoding")
-	websocketKey := r.Header.Get("Sec-WebSocket-Key")
 	if acceptEncHeader == "" {
 		return []string{}
 	}
+	websocketKey := r.Header.Get("Sec-Websocket-Key")
 
 	prefs := []encodingPreference{}
 

modules/caddyhttp/matchers.go

@@ -1368,7 +1368,7 @@ func (m MatchProtocol) Match(r *http.Request) bool {
 func (m MatchProtocol) MatchWithError(r *http.Request) (bool, error) {
 	switch string(m) {
 	case "grpc":
-		return strings.HasPrefix(r.Header.Get("content-type"), "application/grpc"), nil
+		return strings.HasPrefix(r.Header.Get("Content-Type"), "application/grpc"), nil
 	case "https":
 		return r.TLS != nil, nil
 	case "http":