Update test/controllers/sessions_controller_test.rb (#1404)
Update test/controllers/sessions_controller_test.rb (#1404)

Update test/controllers/sessions_controller_test.rb
so that instead of using the obsolete ActionController::TestCase
it is switched to using ActionDispatch::IntegrationTest.
See: https://github.com/rails/rails/issues/22496
I didn't worry about possible overlaps with existing tests;
the existing integration tests are generally long and focus on
important sequences, while these are short tests that check
specific issues.

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>
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