Text in HTML format is sent as HTML in plain/text mail notifications

This fix request #6013

Change-Id: I87adb12965c6d7af4d3145e438bc7e4f9aa73523
Merge commit 'refs/changes/54/1754/1' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Text in HTML format is sent as HTML in plain/text mail notifications

Change-Id: I8974a8b15fba049c6a23317f0ded9d5cd65a2567
This is Tuleap 6.9.99.29

Change-Id: I51226bf9d59d59ef79a7177fb7319a0e24c08baf