ModelForm.__init__() should match argument signature of "standard" forms

Patch which "fixes" ModelForm.__init__()

Updated patch

New patch ensures that a ModelForm without a model or instance throws an error, and tests and documents the now-possible case of a ModelForm working with any of multiple models at runtime.

Discussion for anyone who's only watching here is taking place on django-dev.

I think the below code block which is for form.save method in meta class should be changed.

        # Don't allow more than one Meta model defenition in bases. The fields
        # would be generated correctly, but the save method won't deal with
        # more than one object.
        base_models = []
        for base in bases:
            base_opts = getattr(base, '_meta', None)
            base_model = getattr(base_opts, 'model', None)
            if base_model is not None:
                base_models.append(base_model)
        if len(base_models) > 1:
            raise ImproperlyConfigured("%s's base classes define more than one model." % name)

        # If a model is defined, extract form fields from it and add them to base_fields
        if attrs['_meta'].model is not None:
            # Don't allow a subclass to define a Meta model if a parent class has.
            # Technically the right fields would be generated, but the save 
            # method will not deal with more than one model.
            for base in bases:
                base_opts = getattr(base, '_meta', None)
                base_model = getattr(base_opts, 'model', None)
                if base_model is not None:
                    raise ImproperlyConfigured('%s defines more than one model.' % name)
            model_fields = fields_for_model(opts.model, opts.fields, opts.exclude)
            # fields declared in base classes override fields from the model
            model_fields.update(declared_fields)
            attrs['base_fields'] = model_fields
        else:
            attrs['base_fields'] = declared_fields
        return type.__new__(cls, name, bases, attrs)

(Maybe I should send a patch here, but I am not familiar to patch tool. I will learn it soon...)

Remove those checks, will make mulitiple inherit from ModelForm? possible.

Thanks.

(In [6915]) Fixed #6162. ModelForm?'s init signature now matches Form's. This is a backwards incompatbile change. Based largely on a patch by ubernostrum.