improve function Server.Handler for package ghttp

DONATOR.MD

@@ -71,6 +71,7 @@ We currently accept donation by Alipay/WechatPay, please note your github/gitee
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 |如果🍋|alipay|￥100.00| 错过的奶茶^_^
 |蔡蔡|wechat|￥666.00| gf真强大，让项目省心
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 <img src="https://goframe.org/images/donate.png"/>

net/ghttp/ghttp_server_config.go

@@ -129,6 +129,9 @@ func ConfigFromMap(m map[string]interface{}) (ServerConfig, error) {
 
 // Handler returns the request handler of the server.
 func (s *Server) Handler() http.Handler {
+	if s.config.Handler == nil {
+		return s
+	}
 	return s.config.Handler
 }