Don't use fallback hosts for Realtime token errors

This is a partial fix for #444 (nature of error not considered when
deciding whether to use a fallback host for realtime connections).

Here, I implement just enough of RTN17f to make sure that we don't use a
fallback host in response to a token error. This fixes an intermittent
failure in the test case on connection_failures_spec.rb:1361, which
tests that we correctly handle token errors when resuming a connection
per RTN15c5. This test was intermittently failing because it would
incorrectly try and use a fallback host after receiving the token error,
causing the resume attempt to sometimes fail with "Unable to resume
connection from another site".

I don't have loads of time on to spend on this issue right now, which is
why I'm applying the smallest possible fix. We need to return to #444
and properly consider how to identify the types of errors mentioned in
RTN17f.

Author: lawrence-forooghian

lib/ably/realtime/connection.rb

@@ -690,16 +690,44 @@ def disconnected_from_connected_state?
       end
 
       def use_fallback_if_disconnected?
-        second_reconnect_attempt_for(:disconnected, 1)
+        unless second_reconnect_attempt_for(:disconnected, 1)
+          return false
+        end
+
+        does_error_necessitate_fallback(reason_for_last_time_in(:disconnected))
       end
 
       def use_fallback_if_suspended?
-        second_reconnect_attempt_for(:suspended, 2) # on first suspended state use default Ably host again
+        unless second_reconnect_attempt_for(:suspended, 2) # on first suspended state use default Ably host again
+          return false
+        end
+
+        does_error_necessitate_fallback(reason_for_last_time_in(:suspended))
       end
 
       def second_reconnect_attempt_for(state, first_attempt_count)
         previous_state == state && manager.retry_count_for_state(state) >= first_attempt_count
       end
+
+      # Provides a partial implementation of RTN17f's logic for whether an error necessitates a fallback host.
+      def does_error_necessitate_fallback(error)
+        return false unless error
+
+        # For now we just explicitly exclude token errors. TODO: implement properly in https://github.com/ably/ably-ruby/issues/444
+
+        if error.respond_to?(:status_code) && error.status_code == 401 && error.respond_to?(:code) && Ably::Exceptions::TOKEN_EXPIRED_CODE.include?(error.code)
+          return false
+        end
+
+        true
+      end
+
+      # Returns the error associated with the last state change to the given state (e.g. :disconnected).
+      def reason_for_last_time_in(state)
+        history_item = state_history.reverse.find do |history_item|
+          history_item.fetch(:state) == state
+        end.fetch(:metadata).reason
+      end
     end
   end
 end