Fix #125; make successful outcomes permanently valid
Remove broken spec

The behavior has changed since feac7c8a. When setting the result, if the
current state of the object is valid, it will remain valid indefinitely.
So testing this behavior now requires checking for validity first, then
setting the result. But this isn't any different than just having an
error on the object, so there's no reason for this test.
Fix broken spec

This behavior has changed since 89a01111. The outcome will never change
from valid to invalid. But a new run of the same class should be invalid
after changing the time zone out from under it.
Merge branch 'master' into cache-valid
Update CHANGELOG.md
Add note to documentation