title: WTForms-JSON not working with FormFields
body: <p>Nested forms (FormFields) doesn't get populated with data when I use WTForms-JSON. I can't spot my mistake, see example below.</p>
  
  <pre><code>from flask import Flask, request, jsonify
  from flask_wtf import Form
  from wtforms import TextField, FormField, IntegerField
  from wtforms.validators import InputRequired
  import wtforms_json
  
  app = Flask(__name__)
  app.config["WTF_CSRF_ENABLED"] = False
  wtforms_json.init()
  
  
  class Address(Form):
      street = TextField('street', validators=[InputRequired()])
      number = IntegerField('number', validators=[InputRequired()])
  
  
  class User(Form):
      name = TextField('name', validators=[InputRequired()])
      address = FormField(Address, label='address')
  
  
  @app.route('/', methods=['POST'])
  def why_no_work():
      form = User()
  
      form.from_json(request.json)
      print form.data
  
      if form.validate():
          return jsonify(success='YEAH')
      else:
          return jsonify(errors=form.errors)
  
  
  if __name__ == '__main__':
      app.run(debug=True)
  </code></pre>
  
  <p>I send the following JSON-request</p>
  
  <pre><code>{
      "name": "Alex",
      "address": {
          "street": "Plz Work Street",
          "number": 1337
      }
  }
  </code></pre>
  
  <p>but the print after <code>form.from_json(request.json)</code> reveals that the address object is never populated with data (also, the "appropriate" errors are returned from the route).</p>
  
  <p>Print output:
  <code>{'name': u'Alex', 'address': {'street': u'', 'number': None}}</code></p>
  
  <p>I'm using WTForms 2.0.2, WTForms-JSON 0.2.8</p>
  
  <p>Is this a bug or am I doing something wrong? Thankful for any help!</p>