微信申请退款参数都是按照手册传的，证书也正确但是还是给返回了缺少参数

调用接口https://api.mch.weixin.qq.com/secapi/pay/refund 请求参数为：

<xml>

        <appid>

            <![CDATA[wx7f2421541006**]]>

        </appid>

        <mch_id>1608****0</mch_id>

        <nonce_str>

            <![CDATA[iQ0ddQota9dfXEdHGwa5tD1NJqlzEtBs]]>

        </nonce_str>

        <op_user_id>8888</op_user_id>

        <out_refund_no>

            <![CDATA[T20210416153****588]]>

        </out_refund_no>

        <refund_fee>3</refund_fee>

        <total_fee>3</total_fee>

        <transaction_id>4200000992202104***82</transaction_id>

        <reason>

            <![CDATA[退回]]>

        </reason>

        <sing>

            <![CDATA[CD028852F3D15F6CF45**5F855C41]]>

        </sing>

    </xml>

接口返回值为：

<pre>Array

(

    [return_code] => FAIL

    [return_msg] => 缺少参数

)