Don't start a goroutine when not needed

sync.Once only has the overhead of a single atomic.LoadUint32

Fixes #693

header.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"sync"
 	"sync/atomic"
 	"time"
 )
@@ -1436,7 +1437,7 @@ func isOnlyCRLF(b []byte) bool {
 	return true
 }
 
-func init() {
+func updateServerDate() {
 	refreshServerDate()
 	go func() {
 		for {
@@ -1446,7 +1447,10 @@ func init() {
 	}()
 }
 
-var serverDate atomic.Value
+var (
+	serverDate     atomic.Value
+	serverDateOnce sync.Once // serverDateOnce.Do(updateServerDate)
+)
 
 func refreshServerDate() {
 	b := AppendHTTPDate(nil, time.Now())
@@ -1493,6 +1497,8 @@ func (h *ResponseHeader) AppendBytes(dst []byte) []byte {
 	if len(server) != 0 {
 		dst = appendHeaderLine(dst, strServer, server)
 	}
+
+	serverDateOnce.Do(updateServerDate)
 	dst = appendHeaderLine(dst, strDate, serverDate.Load().([]byte))
 
 	// Append Content-Type only for non-zero responses

server.go

@@ -2474,6 +2474,8 @@ func (s *Server) writeFastError(w io.Writer, statusCode int, msg string) {
 		server = fmt.Sprintf("Server: %s\r\n", s.getServerName())
 	}
 
+	serverDateOnce.Do(updateServerDate)
+
 	fmt.Fprintf(w, "Connection: close\r\n"+
 		server+
 		"Date: %s\r\n"+