Fast path for setting headers.

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

render/text.go

@@ -14,12 +14,12 @@ type String struct {
 	Data   []interface{}
 }
 
-const plainContentType = "text/plain; charset=utf-8"
+var plainContentType = []string{"text/plain; charset=utf-8"}
 
 func (r String) Write(w http.ResponseWriter) error {
 	header := w.Header()
 	if _, exist := header["Content-Type"]; !exist {
-		header.Set("Content-Type", plainContentType)
+		header["Content-Type"] = plainContentType
 	}
 	if len(r.Data) > 0 {
 		fmt.Fprintf(w, r.Format, r.Data...)