We were inconsistent on this one. I consider it particularly useful when
a method starts with a `return` statement.
In other cases, we probably shouldn't have a guard rule in the middle of
a method in any case, but that's a different refactoring.
This way we can easily add a test which will fail if by accident we
change the method to use `Date.today`. Until now using `Date.today`
would only fail if we ran specs in a time zone with a different date.
