`itemRemoved` callback isn't called when entity becomes `deleted`.

[YegorUdovchenko]

itemRemoved callback of a subscription made with Firebase client isn't called when entity becomes deleted.

How to reproduce:

1. Subscribe to updates of a non-existing Task by TaskId.
2. Send CreateTask command.
3. Receive a state of the newly-created task via itemAdded callback.
4. Send DeleteTask command.
5. itemRemoved for the corresponding task is never called

The corresponding test can be found here.

[armiol]

Addressed in 1.x via #197.