Fast path for setting headers.

- No heap allocation
- No indirection
- CanonicalMIMEHeaderKey() is not called

render/xml.go

@@ -13,9 +13,9 @@ type XML struct {
 	Data interface{}
 }
 
-const xmlContentType = "application/xml; charset=utf-8"
+var xmlContentType = []string{"application/xml; charset=utf-8"}
 
 func (r XML) Write(w http.ResponseWriter) error {
-	w.Header().Set("Content-Type", xmlContentType)
+	w.Header()["Content-Type"] = xmlContentType
 	return xml.NewEncoder(w).Encode(r.Data)
 }