[BUGFIX] Handle IDN domain names correctly

Backend fails if using an IDN (international domain name) like täst.org

Change-Id: Iddd0f821bdf61d98f898fd3cdf07b0af83e081d3
Resolves: #20519
Releases: 4.6, 4.5
Reviewed-on: http://review.typo3.org/2528
Reviewed-by: Georg Ringer
Reviewed-by: Kay Strobach
Tested-by: Kay Strobach
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert