Bug 13982: Checkouts table not sorting on correct column

The "Date Due" column for checkouts is using the column index 1 to sort by.
This column is the one that is used for grouping items into today's checkouts
and previous checkouts. This is definitely not the column that should be used.
Instead, we should be using column index 2, which contains the raw unformatted
due date.

Revised Test Plan:
1) Find a patron with no checkouts
2) Check out something as due the beginning on the month.
3) Check out something as due the end of the month.
4) Check out something due tomorrow (hopefully not end of month)
   -- so you should have 2015-04-01, 2015-04-30, and 2015-04-14
      (for example)
5) Attempt to sort by due date
   -- Note the sorting is incorrect
6) Apply this patch
7) Clear your browser cache
8) Reload the page
9) Attempt to sort by due date again
   -- Note the sorting works correctly this time
10) Verify there are no regressions with bug 13908
    -- this will require finding a patron with many checkouts,
       playing with the Circulation system preferences
       previousIssuesDefaultSortOrder and todaysIssuesDefaultSortOrder,
       and understanding what the four different cases mean with
       respect to actually manually sorting the 'Due Date' column.

NOTE: I did not do step 10,
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>

koha-tmpl/intranet-tmpl/prog/en/js/checkouts.js

@@ -191,7 +191,7 @@ $(document).ready(function() {
                     "bVisible": false,
                 },
                 {
-                    "iDataSort": 1, // Sort on hidden unformatted date due column
+                    "iDataSort": 2, // Sort on hidden unformatted date due column
                     "mDataProp": function( oObj ) {
                         var due = oObj.date_due_formatted;