Add and apply RSpec/BeNil rubocop rule

This rule was added in rubocop-rspec 2.9.0.

We were using `be_nil` 50% of the time, and `be nil` the rest of the
time. No strong preference for either one, but IMHO we don't lose
anything be being consistent.

spec/controllers/management/users_controller_spec.rb

@@ -10,8 +10,8 @@ describe Management::UsersController do
       get :logout
 
       expect(session[:manager]).to eq(user_key: "31415926", date: "20151031135905", login: "JJB033")
-      expect(session[:document_type]).to be_nil
-      expect(session[:document_number]).to be_nil
+      expect(session[:document_type]).to be nil
+      expect(session[:document_number]).to be nil
       expect(response).to be_redirect
     end
   end