make adding a duration be done in parts, instead of converting to seconds.

bug 303826, r=dmose

calendar/libical/src/libical/icalduration.c

@@ -310,9 +310,11 @@ int icaldurationtype_is_bad_duration(struct icaldurationtype d)
 struct icaltimetype  icaltime_add(struct icaltimetype t,
 				  struct icaldurationtype  d)
 {
-    int dt = icaldurationtype_as_int(d);
-    
-    t.second += dt;
+    t.second += d.seconds;
+    t.minute += d.minutes;
+    t.hour += d.hours;
+    t.day += d.days;
+    t.day += d.weeks * 7;
     
     t = icaltime_normalize(t);