only check KeepAliveInterval if keep-alives are enabled (#113)

Signed-off-by: Asutorufa <16442314+Asutorufa@users.noreply.github.com>

mux.go

@@ -89,7 +89,7 @@ func VerifyConfig(config *Config) error {
 	if config.AcceptBacklog <= 0 {
 		return fmt.Errorf("backlog must be positive")
 	}
-	if config.KeepAliveInterval == 0 {
+	if config.EnableKeepAlive && config.KeepAliveInterval == 0 {
 		return fmt.Errorf("keep-alive interval must be positive")
 	}
 	if config.MeasureRTTInterval == 0 {