Fix setting session when middleware redirects instead of continuing request flow

[girardinsamuel]

An issue has been observed when using a middleware which set some data in session (flash data or not) as in the example below:

class SomeMiddleware(Middleware):

    def before(self, request, response):
        if condition:
             Session.set("key", "value")
             return response.redirect("/").with_errors("Some error")
        return request

Neither key nor errors are correctly set into session when the redirected route loads because it does not load the session correctly because no cookies had been added to the response for those keys.
It's because Session.save() in the upper SessionMiddleware.after() method had never been called because of the redirect.

So now, .save() is called at each session data update:

• set
• flash
• delete
• pull
• get (only in case of flash data)

This PR fixes it.