Fix a bug in custom signup form classes, which rejected classes that only implemented the new 'signup' method.

django-allauth was updated to use the 'signup' method instead of the 'save' method for custom signup form classes. However, there is one place that didn't get updated and thus it won't work.

Original commit here: https://github.com/pennersr/django-allauth/commit/afc467a539173922b4f11e8f8bc084827ec10ad8#diff-3b9310d6e2c9a729f9c65c16647b3e1aR192