the action="." default on forms has odd behavior with URLs that do not have a trailing slash

I'm not sure if this was deliberate, but having a default action of "." posts to the parent page of the URL doesn't have a trailing slash. For example, if you have a form loaded like this:

```python
@app.route('/a/test/url', methods=('GET', 'POST'))
def expert_finder():
    form = AForm()
    if form.validate_on_submit():
        return redirect('/a/test/url')
    return render_template('test.html', form=form)
```

and something like this in your template

```html
{{ wtf.quick_form(form) }}
```

then when this form is rendered in the html, it actually redirects the POST to the url `/a/test/` instead of `/a/test/url` which is what I expected. Any thoughts on changing the default to `action=""`, which would fix this problem?