Fix if it is set to absolute time, unable to store the problem.

[renyanwei]

Fix if it is set to absolute time, unable to store the problem. GetExpiration(DateTime expiresAt) and GetExpiration(TimeSpan validFor) The meaning of the return is different, there is ambiguity.And Store(StoreMode mode, string key, object value, DateTime expiresAt) cannot be successfully stored。It's as if the data returned by GetExpiration (DateTime, expiresAt) is too large

[enyim]

I don't really get the problem :(

Also, the way you have changed the GetExpires to always return a relative timestamp will cause problems with expirations > 30 days.