Fix for missing Content-Length header, e.g. for cern git service

See discussion here:
https://groups.google.com/forum/?hl=en#!topic/diracgrid-develop/Uw0FAYMo6R0
TL,DR: The cern git service does not send Content-Length header, I can't make distributions from there.
This is a fix for ignoring that header when it is not there.