Depending on the offset between a user's configured timezone and the server
timezone, Date.today may be more or less often wrong from the user's
perspective, leading to things like issues marked as overdue too early or too
late, or yesterday / tomorrow being displayed / selected where 'today' is
intended.

A test case illustrating the problem with Issue#overdue? is included

Patch by Jens Kraemer.

git-svn-id: https://svn.redmine.org/redmine/trunk@15379 e93f8b46-1217-0410-a6f0-8f06a7374b81