bug#17268: tests/guix-download.sh fails due to ISP's wrong-domain redirect

[Taylan Ulrich Bayirli/Kammer]

Hello,

It looks like the test guix-download.sh fails due to unwanted success
when the user's ISP is being "friendly" and redirects to a help page
when a non-existing domain is requested.  Log file:

+ guix download --version
guix download (GNU Guix) 0.6
Copyright (C) 2014 the Guix authors
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
+ guix download http://does.not/exist
accepted connection from pid 8210, uid 1000
starting download of `guix-file.u5jS7M' from `http://does.not/exist'...
following redirection to 
`http://navigationshilfe1.t-online.de/dnserror?url=http://does.not/'...

http://navigationshilfe1.t-online.de/.../dnserror          0.5 KiB transferred
/home/tub/media/src/guix/test-tmp/store/xp2ikq7k1nqjhqwqh8w2zd5vfbwiminn-exist
1r5brk1scchyraimnpvc8lx7lqiiryv31f198hcwv9b72v5mjkfj
+ false
1 operations