Use env[:clearance] instead of env[:session]

Because this test was passing before, but not actually working, I think
this also needs some sort of test to see if

    Clearance::Session.new(env).signed_in?

is true, but I don't have time/knowledge about clearance to do that.


Tested this with `gem 'clearance', path: '/Users/caleb/code/clearance'`
in an app that was using a working BackDoor, and it passed the specs.