Allow incoming subscriptions to have expires 0

Ignore-this: 478605dc8890cdab672e97753123e0fb

darcs-hash:20130125121939-56b4d-5dccf4610e6b23255204b29a65c9eb25f4d6e630
Raised core revision

Ignore-this: 286e6098474804e919fa474244c509c9

darcs-hash:20130125123308-56b4d-0af19b81f621f629304de15a67359436f6d74b5e