Fixed actingAs, withCookies, withHeaders, assertAuthenticated, assertGuest in TestCase and fixed Auth which was not persisting user

[girardinsamuel]

Rewrite of those two PRs #579 and #551.

Fixed

1. Fixed TestCase withHeaders() and withCookies() methods
   headers and cookies were added after the request/response cycle was done, so it was not correct.
   Adding them before, was the solution, but it requires to add them in the WSGI environ object.

2. Fixed actingAs test helper during requests and fixed assertAuthenticated and assertGuest assertions

self.get("/about").assertGuest()
# you can also pass the guard used to check that user is not authenticated
self.get("/about").assertGuest("web")

self.get("/login").assertAuthenticated()
user = User.find(1)
# you can pass the user that should be authenticated
self.get("/login").assertAuthenticated(user)
# you can also pass the guard used to check that user is authenticated
self.get("/login").assertAuthenticated(user, "web")

3. Fixed Auth by persisting user in Auth instance: user is now persisted in the Auth instance, added when logged in and removed when logged out.