Fix OmniAuth CSRF regression test

OmniAuth now rescues the InvalidAuthenticityToken error, so we have to
test that the redirect to GitHub doesn't happen instead (without a CSRF
token, the user is redirected to users/omniauth_callbacks#failure, which
redirects to the root path).

spec/request/omniauth_spec.rb

@@ -28,9 +28,9 @@
       end
 
       it 'requires CSRF token for initial OmniAuth endpoint' do
-        expect {
-          post user_github_omniauth_authorize_path
-        }.to raise_error(ActionController::InvalidAuthenticityToken)
+        post user_github_omniauth_authorize_path
+
+        expect(response).not_to redirect_to(/github\.com/)
       end
     end
   end