Fix _changes heartbeat option

[nickva]

TimeoutFun was already returning {ok|stop, UserAcc} so there was no need to
wrap it another `{ok, ...} tuple.

Also TimeoutFun was calling user with{timeout, _ResponseType} not just timeout, so added a clause to handle that as well.

[davisp]

+1

[nickva]

Replication tests and fabric tests pass.

ReplicationTest
  * test filtered replications - remote-to-remote (592.4ms)
  * test validate_doc_update failure replications - remote-to-remote (229.2ms)
  * test non-admin user on target - remote-to-remote (218.6ms)
  * test replication restarts after filter change - COUCHDB-892 - remote-to-remote (466.9ms)
  * test continuous replication - remote-to-remote (5805.3ms)
  * test simple remote-to-remote replication - remote-to-remote (2147.2ms)
  * test compressed attachment replication - remote-to-remote (1594.9ms)
  * test unauthorized replication cancellation (627.0ms)
  * test replicating attachment without conflict - COUCHDB-885 (255.5ms)
  * test create_target filter option - remote-to-remote (163.3ms)
  * test replicate with since_seq - remote-to-remote (188.8ms)
  * test source database not found with host (13.5ms)
  * test replication cancellation (607.5ms)
  * test replication by doc ids - remote-to-remote (4501.7ms)
  * test non-admin or reader user on source - remote-to-remote (133.9ms)