Before initiating a request, RBTools checks if the cache contains an

entry expected to be up-to-date. This compares the value in Expires or

Cache-Control: max-age=... to the local time, and if it's too old, the

caller is free to make the request.
The problem is that this code had a bad default in the case where no

max-age was stored. If it was None, this function defaulted to

"up-to-date", preventing requests from going through.
This breaks with Review Board 6, because the headers being sent in the

API no longer include an Expires or Cache-Control by default,

causing a None value to be stored, and triggering the broken behavior.
Due to this, users will need to upgrade to RBTools 4.1 or higher for

Review Board 6, in order to avoid caching issues.

Unit tests pass.
Verified this fixed the caching bugs discovered since improving caching

behavior in Review Board 6.