Fix broken form.save() call in DjangoFormMutation.perform_mutate

[ruohola]

Django's plain (non-model) forms don't have the save method, so
calling this would just result in an AttributeError before this
change.

Resolves #1152

---

Didn't want to just remove the form.save() line completely, since the tests at least "document" it as a valid usage to pass a ModelForm as the form_class of a plain DjangoFormMutation:

graphene-django/graphene_django/tests/mutations.py

Lines 9 to 11 in 80ea51f

	class PetFormMutation(DjangoFormMutation):
	class Meta:
	form_class = PetForm

(PetForm is a forms.ModelForm here)

[zbyte64]

👍 seems sensible to me