title: How to use Schema.from_dict() for nested dictionaries?
body: <p>I am trying to create a Schema class using nested dictionaries that has some list as elements. However when I do a dumps() Only the top level elements are dumped.</p>
  
  <p>Have a rest api that returns a list of certain things,eg. list of users. but the schema is such that certain aggregate details are sent at the top level, the data looks something like this. This is what i am expecting as output:</p>
  
  <pre><code>   {
          "field1": 5,
          "field2": false,
          "field3": {
              "field4": 40,
              "field5": [
                          {
                              "field6": "goo goo gah gah",
                              "field7": 99.341879,
                              "field8": {
                                  "field9": "goo goo gah gah",
                                  "field10": "goo goo gah gah"
                          }
               }]
           }
      }  
  </code></pre>
  
  <p>Heres my code:</p>
  
  <pre><code>MySchema = Schema.from_dict(
     {
          "field1": fields.Int(),
          "field2": fields.Bool(),
          "field3": {
              "field4": fields.Int(),
              "field5": [
                          {
                              "field6": fields.Str(),
                              "field7": fields.Float(),
                              "field8": {
                                  "field9": fields.Str(),
                                  "field10": fields.Str()
                          }
               }]
           }
      }            
  )
  
  #Then use it like:
  
  response = MySchema().dumps(data)
  </code></pre>
  
  <p>Actual result:</p>
  
  <pre><code>"{\"field1\": 5, \"field2\": false}"
  </code></pre>