Realtime connection does not use fallback hosts in custom environment

[lawrence-forooghian]

For example, change the test test__091__Connection__Host_Fallback__every_connection_is_first_attempted_to_the_primary_host_realtime_ably_io so that it uses the sandbox environment (i.e. write options.environment = "sandbox"). You will see that the client does not attempt to connect to any fallback hosts. I think that, per RTN17c and RSC15g2, it should do.

Fallback hosts do seem to be working correctly for custom environments for REST connections; see the tests for RSC15g2.

(Note that I'm referring to the pre-1ef8d1a version of the spec, which deprecated the spec points I'm referring to and introduced the endpoint client option, which we haven't implemented yet.)

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