title: WTForms: FieldList of FormField can't load nested data
body: <p>I have a custom field inside a FormField inside a FieldList: <strong>locations</strong></p>
  
  <pre><code>class LocationForm(Form):
      id = HiddenField('id')
      title = StringField(_l('Title'), [Required()])
      location = CoordinatesField(_l('Coordinates'))
  
  class ProjectForm(Form):
      title = StringField(_l('Title'))
      manager = StringField(_l('Manager'))
      description = StringField(_l('Description'))
      locations = FieldList(FormField(LocationForm), min_entries=1)
  </code></pre>
  
  <p>This form when submited is saved to an object like this:</p>
  
  <pre><code>document = {
      'title': unicode,
      'description': unicode,
      'manager': unicode,
      'locations': [{
          'id': uuid.UUID,
          'title': unicode,
          'location': {'coordinates':[float], 'text':unicode}
          }],
      }
  </code></pre>
  
  <p>When I try to load the data in to the form for a GET handler, 
  everything <strong>but the locations</strong> loads fine:</p>
  
  <pre><code>f = form(MultiDict(document))
  f.locations.data
  &gt;&gt; {'id':'','title':'','location':''}
  </code></pre>
  
  <p>I did some debugging and found that WTForms while loading the document's data in to the form searches for 'locations-0-location' but MultiDict() but that keys doesn't exists. MultiDict doesn't convert a list of dictionaries to the key 'locations-i-...'.</p>
  
  <p>What is the right way to make a WTForm for such a nested data structure?</p>