[INTERNAL] sender JID was not set for new sendData method

Forget to set the sender JID in the transfer description
for the new sendData method introduced with commit
4da0ecf5507c5267a832b279832bd808c7e1d459 .

Change-Id: I016c1010086d13d23fd012bf93c6bfab1df1e73d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/950
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>