title: Flask Wtform calling both FieldList and FormField validate() causes error
body: <p>so I have this code</p>
  
  <pre><code>class ItemPurchaseForm(wtforms.Form):
      purchase_price = IntegerField(label='Purchase Price: ',
                                    validators=[InputRequired()])
  
      def validate(self, *args, **kwargs):
          if not super().validate():
              self.purchase_price.errors += (super().errors,)
              return False
          #.... do other validations....
  
  class PurchaseTransactionForm(Form):
      yyyy = IntegerField(label='Transaction Year',
                          validators=[InputRequired()])
  
      transaction_items = FieldList(FormField(ItemPurchaseForm),
                                    label='Purchased items',
                                    min_entries=1)
      submit_button = SubmitField(label='Add new purchase transaction')
  
      def validate(self, **kwargs):
  
          if not super().validate():
              self.yyyy.errors += (super().errors, 'super not validated')
              return False
  
           #.... do some other validation
  </code></pre>
  
  <p>As you can see there is a <code>FieldList(FormField(...))</code>, so I followed this <a href="https://stackoverflow.com/questions/15649027/wtforms-csrf-flask-fieldlist">answer</a> to fix previous error caused by CRSF field. And now if I have <code>def validate()</code> in <code>ItemPurchaseForm</code> it will give me error, and the error is just this: <code>{'transaction_items': [{}]}</code>.</p>
  
  <p>If I remove the <code>def validate()</code> from <code>ItemPurhcaseForm</code>, everything works well. Is there any reason why that causes error? I did read this <a href="http://wtforms.readthedocs.org/en/latest/fields.html#wtforms.fields.Field.validate" rel="nofollow noreferrer">http://wtforms.readthedocs.org/en/latest/fields.html#wtforms.fields.Field.validate</a>, so does it mean that <code>ItemPurchaseForm</code> is a Subfield?</p>