title: Empty struct on Unmarshall JSON with unknown key
body: <p>I'm getting from and API a JSON like the following:</p>
  
  <pre><code>{
    "unknownkey" : {
      "sum" : 7030.76636,
      "low" : 6787.05692,
      "avg" : 0.00014
    }
  }
  </code></pre>
  
  <p>First key is unknown - it changes, but the struct inside holds always the same keys.
  Following the discussion <a href="https://stackoverflow.com/questions/33436730/unmarshal-json-with-some-known-and-some-unknown-field-names">here</a> I created the these <code>structs</code>:</p>
  
  <pre><code>type Order struct {
    Result Result `json:"-"`
  }
  
  type Result struct {
    Sum        float64    `json:"sum"`
    Low        float64    `json:"low"`
    Avg        float64      `json:"avg"`
  }
  </code></pre>
  
  <p>Which I try to unmarshall it like this:</p>
  
  <pre><code>var order Order
  json.Unmarshal([]byte(body), &amp;order)
  </code></pre>
  
  <p>but I just get an empty struct:</p>
  
  <pre><code>{Result:{Sum:0 Low:0 Avg:0}}
  </code></pre>