Fix request #6429 - PUT /milestones/123/content doesn't change the order of elements

Change-Id: Ied61898a0e0292aba486513ead3c7c1c243a4798
Merge commit 'refs/changes/74/2074/2' of http://gerrit.tuleap.net/tuleap into HEAD

* http://gerrit.tuleap.net/tuleap:
  Fix request #6429 - PUT /milestones/123/content doesn't change the order of elements

Change-Id: Ic24a564f49bc27f36b61e22f9f1299bd3afa3405
This is Tuleap 6.11.99.8

Change-Id: I9f8c92693f793bd6767518376e8a1a608260880d