Ensure single subscription to Gate opening events

[glopesdev]

This PR fixes the behavior of the Gate operator to be consistent with common language patterns re. subscription. The Gate operator in Rx can be called either with a factory overload, where a new subscription is created for each gate trigger, or with a single subscription to an event sequence, where each notification triggers the gate.

The latter overload is more consistent with the rules of similar operators such as WindowTrigger. In general one can use the Defer operator if the former behavior is required. Alternatively, a new variant of the Gate operator can be exposed in the future which surfaces the factory overload through a nested operator.

Fixes #1206