Time marshalling is incorrect

https://github.com/tdtran/resque-uniq/pull/14 is broken but has exposed the root cause of the problem.

JsonEncoder is doing the marshalling in resque but it doesn't handle Time objects correctly (the sender sends a Time object but the reciever gets a String).

This means the lock on the enqueueing side does to_i on the Time but the lock on the performing side uses the string version of the Time.