Handle no status in FormatCloseMessage

Return empty message for CloseNoStatusReceived. This status indicates
that the message is empty, so make it so. Because it's illegal to send
CloseNoStatusReceived, this change should not break a correct
application.

conn.go

@@ -1061,10 +1061,7 @@ func (c *Conn) CloseHandler() func(code int, text string) error {
 func (c *Conn) SetCloseHandler(h func(code int, text string) error) {
 	if h == nil {
 		h = func(code int, text string) error {
-			message := []byte{}
-			if code != CloseNoStatusReceived {
-				message = FormatCloseMessage(code, "")
-			}
+			message := FormatCloseMessage(code, "")
 			c.WriteControl(CloseMessage, message, time.Now().Add(writeWait))
 			return nil
 		}
@@ -1142,7 +1139,14 @@ func (c *Conn) SetCompressionLevel(level int) error {
 }
 
 // FormatCloseMessage formats closeCode and text as a WebSocket close message.
+// An empty message is returned for code CloseNoStatusReceived.
 func FormatCloseMessage(closeCode int, text string) []byte {
+	if closeCode == CloseNoStatusReceived {
+		// Return empty message because it's illegal to send
+		// CloseNoStatusReceived. Return non-nil value in case application
+		// checks for nil.
+		return []byte{}
+	}
 	buf := make([]byte, 2+len(text))
 	binary.BigEndian.PutUint16(buf, uint16(closeCode))
 	copy(buf[2:], text)