fix(emitter): off() ignoring handler check when one listener remains

[tassoevan]

Proposed changes (including videos or screenshots)

For any Emitter instance, if there is only one event handler attached to some event x, calling emitter.off('x', handler) will wipe that handler regardless of handler being the one.

Issue(s)

Further comments

it('should remove only the specified handler', () => {
    const handler = jest.fn();
    const unusedHandler = jest.fn();

    emitter.on('test', handler);
    emitter.off('test', unusedHandler);

    emitter.emit('test');

    expect(handler).toHaveBeenCalledTimes(1); // Actual: 0
    expect(unusedHandler).toHaveBeenCalledTimes(0);
  });