DateTimeInterval#duration returns a wrong value

[hackugyo]

DateTimeInterval#duration returns a negative value when it is initialized with values.
This means DateTimeInterval#end is not equal to the init value.
(iOS10 DateInterval can return correct values.)

[malcommac]

Thank you, it will be part of 4.1.8 release

[malcommac]

I think we have a problem here, I think I've made a mistake; can you explain me why we can't have DateTimeInterval with negative values?
@hackugyo

[hackugyo]

I've checked #480 and now think we can allow DateTimeInterval with negative values!
The most reason I made PR #468 is from the inconsistency of DateTimeInterval#end. Your 00f9403 solves this inconsistency.
@malcommac

[leviathan]

let startDate = "10:00"
let endDate = "12:00"

let dateDiff = startDate - endDate

>> raises Exception now in DateTimeInterval #67
>> precondition(end >= start, "Reverse intervals are not allowed")

why should it not be allowed to have a negative time interval ?!

[malcommac]

@leviathan I've fixed that, there is not any valid reason to keep this condition. Now DateTimeInterval supports negative interval. See the commit above.