title: golang json decode with field name
body: <p>For following JSON</p>
  
  <pre><code>{
      "Jon": {
         "Age": 15
      },
      "Mary": {
         "Age": 17
      } 
  }
  </code></pre>
  
  <p>how can i map it into golang struct, normally, the structure will be</p>
  
  <pre><code>type Person struct {
      Name string `json:??,string`
      Age int `json:Age, int`
  }
  </code></pre>
  
  <p>as the json field name is the attribute of struct, thank you in advance.</p>