If the user form is submitted without authentication, redirect to login but keep entered data

[stephanegigandet]

One user reports that after spending a lot of time filling data in the product edit form, he was redirected to the login page, which made him lose all the edits. The back button did not allow to get back the entered values.

It would be great if we could somehow keep the edits so that they are automatically applied after authentification. It is a bit tricky as we can't redirect POST requests.