fix test_download_db

This should probably fix the failure I saw at http://172.19.49.85/view/arakoon-git/job/arakoon-amplidata-git-system-tests-QUICK/31/testReport/junit/arakoon_system_tests.server.quick/system_tests_basic/test_download_db/ .
If you have better ideas, which are applicable here, then please share.