Bug #1505: Fix division by zero calculating margin with deadline

The margin with deadline is divided by the order length in days. If the order
starts and finish in the same day that length was been counted as 0 when it is
really 1.

FEA: ItEr76S04BugFixing

libreplan-webapp/src/main/java/org/libreplan/web/dashboard/DashboardModel.java

@@ -223,7 +223,7 @@ public class DashboardModel implements IDashboardModel {
         }
         TaskGroup rootTask = getRootTask();
         Days orderDuration = Days.daysBetween(rootTask.getStartAsLocalDate(),
-                rootTask.getEndAsLocalDate());
+                rootTask.getEndAsLocalDate().plusDays(1));
 
         LocalDate deadLineAsLocalDate = LocalDate.fromDateFields(currentOrder
                 .getDeadline());