Fixed broken Preview (displaying the old article content)

The Preview was currently broken due to [df27e51032](https://github.com/benjaoming/django-wiki/commit/df27e51032). It was displaying the old content everytimes.

I also verified the correctness of reverting the change by debugger, debug printing and by reading the related code. The header of the class FormEdit is:

    class EditForm(forms.Form, SpamProtectionMixin):
        def __init__(self, request, current_revision, *args, **kwargs):

The added custom arguments are `request` and `current_revision` that are never passed to `super(EditForm...)...`, but the `*args` should be passed completely because the first argument of them is the standard `data` object or dictionary necessary for forms.

I wrote a test for this bug and for some basic functionality of the wiki. (The previous demo test that 1+1==2 is removed :-)