Remove all `include ActiveSupport::Testing::MethodCallAssertions`
in actioncable's tests since we can do it only in `ActionCable::TestCase`
in order to prevent code duplication.
We use the same approach for other modules of Rails.
We have defined `ActionCable::TestCase` in `actioncable/test/test_helper.rb`
that we can use in order to prevent code duplication and build common
interface for actioncable's test.
Similar to the channel streaming side, these values must be strings for
ActionCable to behave as expected. The conversion will allow users to
send string-convertible values and get the expected behavior.