Use http.Header methods instead of being clever

golang/go@1e814df tried to avoid
unnecessary calls to CanonicalHeaderKey in the standard library, but it
is not clear that there is any meaningful performance gain, nor is it
used consistently. This change mostly reverts the manual manipulation
of the http.Header map by using the provided methods instead.

gzip.go

@@ -147,13 +147,13 @@ func (w *responseWriter) startGzip() (err error) {
 	h := w.Header()
 
 	// Set the GZIP header.
-	h["Content-Encoding"] = []string{"gzip"}
+	h.Set("Content-Encoding", "gzip")
 
 	// if the Content-Length is already set, then calls
 	// to Write on gzip will fail to set the
 	// Content-Length header since its already set
 	// See: https://github.com/golang/go/issues/14975.
-	delete(h, "Content-Length")
+	h.Del("Content-Length")
 
 	// Write the header to gzip response.
 	w.ResponseWriter.WriteHeader(w.code)
@@ -218,7 +218,7 @@ func (w *responseWriter) inferContentType(b []byte) {
 	}
 
 	// It infer it from the uncompressed body.
-	h["Content-Type"] = []string{http.DetectContentType(b)}
+	h.Set("Content-Type", http.DetectContentType(b))
 }
 
 func (w *responseWriter) shouldPassThrough() bool {
@@ -324,8 +324,7 @@ func (h *handler) pool() *sync.Pool {
 }
 
 func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	hdr := w.Header()
-	hdr["Vary"] = append(hdr["Vary"], "Accept-Encoding")
+	w.Header().Add("Vary", "Accept-Encoding")
 
 	var acceptsGzip bool
 	for _, spec := range header.ParseAccept(r.Header, "Accept-Encoding") {