fix duration

Author: wxiaoguang

src/duration.ts

@@ -182,6 +182,9 @@ export function roundToSingleUnit(duration: Duration, {relativeTo = Date.now()}:
     } else if (monthsDiff < 11) {
       months = monthsDiff
       years = 0
+    } else if (monthsDiff === 11 && years === 0) {
+      months = 0
+      years = 1 // the old behavior: "11 months" is rounded to "1 year"
     } else {
       months = 0
       years = yearDiff * sign