[sebxt]

get_many and gets_many are typed as taking Iterable[Key], but _fetch_cmd iterates the keys twice. With a generator the second pass sees nothing, so any hit raised KeyError (or, with ignore_exc=True, the call silently returned {}), and an empty generator sent a bare get command that the server rejects.

This converts keys to a list at the top of get_many and gets_many, before the emptiness check. PooledClient delegates to these methods and is fixed as well.

Tests: added TestClient tests for get_many and gets_many with a generator and with an empty iterator. The full unit and integration suite passes.

Fixes #667