json.net xml转换为json格式时，如何将指定节点转换成数组

 using System.Xml.Linq;
 using Newtonsoft.Json;

 Response.ContentType = "application/json";
 XDocument xdoc = XDocument.Load(path);
 Response.Write(JsonConvert.SerializeXNode(xdoc));

xml 片段 ：

<specialty nameCN="电测">
        <step>
            <signer staffID="800706" nameCN=""><![CDATA[]]></signer>
        </step>
        <step>
            <signer staffID="090477" nameCN=""><![CDATA[]]></signer>
        </step>
    </specialty>
    <specialty nameCN="节能">
        <step>
            <signer staffID="800608" nameCN=""><![CDATA[]]></signer>
            <signer staffID="800808" nameCN=""><![CDATA[]]></signer>
        </step>
        <step>
            <signer staffID="800602" nameCN=""><![CDATA[]]></signer>
            <signer staffID="800803" nameCN=""><![CDATA[]]></signer>
        </step>
    </specialty>

输出 json 结果：

{
  "@nameCN": "电测",
  "step": [
    {
      "signer": {
        "@staffID": "800706",
        "@nameCN": "",
        "#cdata-section":
      }
    },
    {
      "signer": {
        "@staffID": "090477",
        "@nameCN": "",
        "#cdata-section":
      }
    }
  ]
},
{
  "@nameCN": "节能",
  "step": [
    {
      "signer": [
        {
          "@staffID": "800608",
          "@nameCN": "",
          "#cdata-section":
        },
        {
          "@staffID": "800808",
          "@nameCN": "",
          "#cdata-section":
        }
      ]
    },
    {
      "signer": [
        {
          "@staffID": "800602",
          "@nameCN": "",
          "#cdata-section":
        },
        {
          "@staffID": "800803",
          "@nameCN": "",
          "#cdata-section":
        }
      ]
    }
  ]
}

上面的结果 用红色标记出来的就是差别，step下有多个signer节点时，输出结果signer是数组，

只有1个signer节点 输出signer不是数组，如何在只有一个signer节点时也输出为数组