Fix for #1832

1. Removed method `GetSagaTypeToStartIfMessageNotFoundByFinder` which was not being called at all!
2. `ShouldMessageStartSaga` will return true if the message type that is tied to the saga is an assignable type to the message type being evaluated.
3. Added an acceptance test for this scenario.  

Also need to investigate why the above method is no longer being called and if there are any other side effects!! 

@andreasohlund @SimonCropp - code review please!

PR against master to show just the changes being made for this fix!