title: Value: xx must be an instance of <class 'dict'> with EmbeddedField and ModelForm
body: <p>Trying use <code>EmbeddedField</code> of Djongo but without any success.</p>
  <p>Output error: <code>Value: en must be an instance of &lt;class 'dict'&gt;</code></p>
  <pre><code>from djongo import models
  from django import forms
  
  
  class Language(models.Model):
      CODE_LANG_CHOICES = (
          ('en', 'English'),
          ('pt', 'Portuguese'),
      )
  
      DIRECTION_CHOICES = (
          ('ltr', 'Left-to-right'),
          ('rtl', 'Right-to-left'),
      )
  
      code = models.CharField(max_length=2, choices=CODE_LANG_CHOICES, default='en')
      direction = models.CharField(max_length=3, choices=DIRECTION_CHOICES, default='ltr')
  
      def __str__(self):
          return f'{self.code}'
  
      class Meta:
          abstract = True
  
  
  class LanguageForm(forms.ModelForm):
      class Meta:
          model = Language
          fields = ('code', 'direction')
  
  
  class Campaign(models.Model):
      _id = models.ObjectIdField()
      language = models.EmbeddedField(model_container=Language, model_form_class=LanguageForm)
      objects = models.DjongoManager()
  
      def __str__(self):
          return f'{self.name}'
  </code></pre>
  <p>Preview error on Django Admin:</p>
  <p><a href="https://i.stack.imgur.com/vTtAT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/vTtAT.png" alt="enter image description here" /></a></p>
  <p>My <code>requirements.txt</code> file:</p>
  <pre><code>Django==2.2.16
  sqlparse==0.2.4
  git+https://github.com/nesdis/djongo.git@master#egg=djongo
  djangorestframework==3.12.1
  </code></pre>
  <p>Python 3.6.12
  MongoDB 3.6</p>