Leader can treat observers as quorum members

In short: it seems leader can treat observers as quorum members.

Steps to repro:

1. Server configuration: 3 voters, 2 observers (attached).
2. Bring up 2 voters and one observer. It's enough for quorum.
3. Shut down the one from the quorum who is the follower.

As I understand, expected result is that leader will start a new election round so that to regain quorum.
But the real situation is that it just says goodbye to that follower, and is still operable. (When I'm shutting down 3rd one – observer – leader starts trying to regain a quorum).

(Expectedly, if on step 3 we shut down the leader, not the follower, remaining follower starta new leader election, as it should be).