ThenByDescending(keySelector: T): OrderedEnumerable<T>;

replacing 
ThenByDescending(keySelector: T): OrderedEnumerable<T>; 
with 
ThenByDescending(keySelector: string): OrderedEnumerable<T>;
Merge pull request #3231 from dardino/patch-1

ThenByDescending(keySelector: T): OrderedEnumerable<T>;