生成unsigned_str：

app_id=800194811070&body=测试订单&currency=CNY&merchant_id=1900019481&notify_url=ttt&out_order_no=157181258944223444&payment_type=direct&product_code=pay&sign_type=MD5&subject=测试订单&timestamp=1571812589000&total_amount=1800&trade_time=1571812589000&trade_type=H5&uid=3444&valid_time=300&version=2.0&wx_type=MWEB&wx_url=https://wx.tenpay.com