added test for read models based on all events
added test for read models based on all events

* the subscribe_to_all_events method was untested
* increased mutation coverage from 93.48% to 95.34%

mlomnicki/rails_event_store,arkency/rails_event_store,mlomnicki/rails_event_store,arkency/rails_event_store,mpraglowski/rails_event_store,RailsEventStore/rails_event_store,RailsEventStore/rails_event_store,RailsEventStore/rails_event_store,RailsEventStore/rails_event_store,arkency/rails_event_store,arkency/rails_event_store,mpraglowski/rails_event_store,mpraglowski/rails_event_store,mpraglowski/rails_event_store