Improve coverage for user return to

[josevalim]

I accidentally broke the user_return_to implementation and I was not able to find this mistake for a couple weeks, so I suggest adding this controller test to help avoid others accidentally breaking it too:

# user_session_controller_test
test "logs the user in with return to", %{conn: conn, user: user} do
  conn =
    conn
    |> init_test_session(user_return_to: "/foo/bar")
    |> post(Routes.user_session_path(conn, :create), %{
      "user" => %{
        "email" => user.email,
        "password" => valid_user_password(),
        "remember_me" => "true"
      }
    })

  assert redirected_to(conn) == "/foo/bar"
end

[josevalim]

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