fix bug in simplemooc

fix.py

# old
@login_required
def undo_enrollment(request, slug):
    course = get_object_or_404(Course, slug=slug)
    enrollment = get_object_or_404(
        Enrollment,
        user=request.user,
        course=course
    )
    if request.method == 'POST':
        enrollment.delete()
        messages.success(request, 'Sua inscrição foi cancelada com sucesso.')
        return redirect('accounts:dashboard')
    template = 'courses/undo_enrollment.html'
    context = {
        'enrollment': enrollment,
        'course': course
    }
    return render(request, template, context)



# new
@login_required
def undo_enrollment(request, slug):
    course = get_course_by_instance(slug, Course, CourseTRB)
    content_type = ContentType.objects.get_for_model(course)
    enrollment = get_object_or_404(
        Enrollment,
        user=request.user,
        content_type=content_type,
        object_id=course.id
    )
    if request.method == 'POST':
        enrollment.delete()
        messages.success(request, 'Sua inscrição foi cancelada com sucesso.')
        return redirect('accounts:dashboard')
    template = 'courses/undo_enrollment.html'
    context = {
        'enrollment': enrollment,
        'course': course
    }
    return render(request, template, context)