title: BooleanField default value not set when creating a model instance
body: <p>When creating an instance of a model having a <code>BooleanField</code> <code>my_boolean_field</code> with a <code>default</code> set to <code>True</code>, I  get an error:</p>
  
  <pre><code>my_boolean_field is required
  </code></pre>
  
  <p>Shouldn't it be set to the default value?</p>
  
  <p><b>models.py</b></p>
  
  <pre><code>class MyModel(User):
  
      my_boolean_field = models.BooleanField(default=False)
  </code></pre>
  
  <p><b>admin.py</b></p>
  
  <pre><code>class MyModelCreationForm(UserCreationForm):   
  
      my_boolean_field = forms.BooleanField(initial=False)
  
      class Meta:
          model = User
  
  class MyModelChangeForm(UserChangeForm):
  
      my_boolean_field = forms.BooleanField(initial=False)
  
      class Meta:
          model = User
  
  
  class MyModelAdmin(UserAdmin):
  
      form = MyModelChangeForm
      add_form = MyModelCreationForm
  
      list_filter = ()
      list_display = ('username', 'my_boolean_field')
  
      fieldsets = (
          (None, {'fields': ('username', 'my_boolean_field', 'password' )}),
      )
  
  
      add_fieldsets = (
          (None, {
              'classes': ('wide',),
              'fields': ('username', 'my_boolean_field', 'password1', 'password2')}
          ),
      )
  
      def get_form(self, request, obj=None, **kwargs):
          form = super(MyModelAdmin, self).get_form(request, obj, **kwargs)
          if obj==None:
              form.base_fields['username'].widget.attrs['autocomplete'] = 'off'
              form.base_fields['password1'].widget.attrs['autocomplete'] = 'off'
              form.base_fields['password2'].widget.attrs['autocomplete'] = 'off'
          return form
  
  samsic_site.register(MyModel, MyModelAdmin)
  </code></pre>