Add support for digest_size_threshold = 0
This patch results in no more than one digest being sent per day if digest_size_threshold is set to zero.
This behavior is documented at http://www.gnu.org/software/mailman/mailman-admin/node19.html: "Set this variable to zero to specify that there is no size threshold, in which case no more than one digest will be sent out per day."
ToDigest.py does not currently implement this behavior. Under the current code, a threshold of zero will always be exceeded, so digest mode will not result in any reduction of traffic for recipients (and will in fact increase message size.) This patch expands the test to check the zero case.