redownload badly cached files by download utility

[Łukasz Nowak]

If file in buildout:download-cache is incorrectly downloaded (eg. md5sum does not match) it would be good to have automatic redownload try.

Proposed scenario:

File X exists in cache as /download-cache/X

Its md5sum value is A, but passed to download utility md5sum value is B.

Buildout checks /download-cache/X, which is wrong (does not match B), so it silently downloads again passed url and rechecks md5sum. In case of failure raises exception related with checksum mismatch. In case of success continues.

[Łukasz Nowak (ionowaklukasz)]

I did fast implementation, only to show where it might be done eventually.

[Łukasz Nowak (ionowaklukasz)]

Please see attached updated version of patch and a test. I needed to redownload the good file to stabilise the test result, maybe I§ shall prove working of this feature in different part of this doctest?