fixup! download: clean-up, fix, optimization

To be squashed with commit 4ccb79e9.
2 jobs for master in 0 seconds (queued for 1 second)
Status Job ID Name Coverage
  External
passed SlapOS.Buildout.UnitTest-Master.Python2

00:07:22

passed SlapOS.Buildout.UnitTest-Master.Python3

00:11:58