quickfix around the bug--"content-length" error for a file of 30Mb

See http://git-annex.branchable.com/tips/googledriveannex/#comment-a53cb8344acc3558d0b20e5d726c6004
wouldn't be better ```int(con.headers.get('content-length', 0))```, or guard with try/except so to maintain original functionality?