title: Not a valid choice: Can't select ReferenceProperty value at SelectField wtform
body: <p>i try to use ReferenceProperty at form for create/edit Entry but nothing happen.</p>
  
  <p>i have:</p>
  
  <pre><code>class Type(db.Model):
      name        = db.StringProperty()
  
  
  
  class Entry(db.Model):
      type            = db.ReferenceProperty(Type, required=False)
  
  class EntryForm(Form):
      _type_list = []
      for type in Type.all():
          _type_list.append((type.key(),type.name))
      type            = fields.SelectField(u'Type of entry', choices = _type_list)
  </code></pre>
  
  <p>and edit handler:</p>
  
  <pre><code>def post(self, **kwargs):
      self.form = EntryForm(self.request.form)
      if self.form.validate():
          values = {
              'type': models.Type.get_by_key_name(self.form.type.data).key(),
          }
          entry = Entry(**values)
          entry.put()
  </code></pre>
  
  <p>but i always have: Not a valid choice</p>
  
  <p>Does enyone know how to work with ReferenceProperty  in wtforms SelectField or may you have working sample for this?</p>