select_default_view cancel button does not work

[sverbois]

There are two problems with the @@select_default_view cancel button.

• when you cancel the selection without choice, you are redirected to the form view instead of the context view
• when you cancel the selection with a choice, the choice is saved instead of being cancelled

The problem comes from the call method here : https://github.com/plone/plone.app.content/blob/master/plone/app/content/browser/selection.py#L40