improve bufferWhileBy test
improve bufferWhileBy test

Since both `send(prop(), [true, false])` and `send(prop(), [false])` create a property with current value `false`.

kelsadita/kefir,AgentME/kefir,AgentME/kefir,elsehow/kefir,elsehow/kefir,pixelpicosean/kefir,pixelpicosean/kefir,kelsadita/kefir,lautis/kefir,lautis/kefir