Check that request.session exists when testing for impersonation

[gadybadger]

I ran into an issue with loginas where the context processor for adding the is_impersonated variable to templates can conflict with installed middleware.

This seems to be caused by the request not having a session property in all cases after passing through all of the middleware, and before passing through all of the context processors.

The fix here is to simply check if the session property exists, otherwise the is_impersonated_session method will always return False.

[skorokithakis]

Looks good, thank you!