Use Timespan.TotalSeconds instead of Timespan.Seconds

The timespan calculation applied as part of the fix for issue NCBC-485 is incorrect.

// NCBC-485 send small TTLs (less than 30 days) in seconds, not EPOCH.
if(validFor < TimeSpan.FromSeconds(((60*60) * 24)*30))

{ return (uint)validFor.GetValueOrDefault().Seconds; }

Note that this should be: return (uint)validFor.GetValueOrDefault().TotalSeconds;

In most cases where a "round" Timespan is used, 0 will be returned as a result of using the "Second" property.