title: marshalling dictionary with variable keys in flask restful /plus
body: <p>I am building an API using flask-restful. I am also using flask-resfulplus for generating swagger documentation. I want to return a dictionary of items where the key is going to vary depending on the item. My model looks like this:</p>
  
  <pre><code>item = api.model('Item',{
              'item':fields.Integer()}) &lt;- This is wrong
  
  
  ItemModel = api.model('ItemsList', {
          '_header': fields.Nested(_header),
          'items':fields.Nested(item)
  })
  </code></pre>
  
  <p>Note that I tried a few variations of this, but nothing seems to work; this is just the latest interation. </p>
  
  <p>And the response I am looking for is something like this. </p>
  
  <pre><code>{
          '_header':{} &lt;-This works fine
          'items': {
           'item1':5,
           'item2':2
           }
      }
  </code></pre>
  
  <p>Item in the items dictionary will have a different key for the item and the count as the value. </p>
  
  <p>I've tried setting the item field to field.Raw() and it works fine, but then it doesn't get displayed in the swagger documentation.</p>
  
  <p>Note that I do not want to return a list of dictionaries, which I have working. </p>
  
  <p>Thanks</p>