Failing test - authUrl JWT tests depend on external host

[mattheworiordan]

rspec './spec/acceptance/realtime/auth_spec.rb[1:2:11:4:1]' # Ably::Realtime::Auth using MsgPack protocol when using JWT when JWT token expires client disconnects, a new token is requested via auth_callback and the client gets reconnected

This test intermittently fails possibly because it relies on a 3rd party endpoint echo.ably.io. That is not necessary given we can serve our own web server endpoint.

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