zsync does not respect proxy settings

Trying to get systemrescuecd-3.7.1-beta006.iso via zsync, while having set http_proxy, https_proxy and ftp_proxy:

--- snip
$ zsync -i systemrescuecd-x86-3.7.1-beta004.iso http://beta.sysresccd.org/systemrescuecd-x86-3.7.1-beta006.iso.zsync
#################### 100.0% 0.0 kBps DONE

reading seed file systemrescuecd-x86-3.7.1-beta004.iso: ****************************************************************************************************************************************************************************************************************************************************************************************************************************************************Read systemrescuecd-x86-3.7.1-beta004.iso. Target 80.3% complete. ********
reading seed file systemrescuecd-x86-3.7.1-beta006.iso.part: ***********************************************************************************************************************************************************************************************************************************************************************************************************************************************Read systemrescuecd-x86-3.7.1-beta006.iso.part. Target 80.3% complete. ********
downloading from http://beta.sysresccd.org/systemrescuecd-x86-3.7.1-beta006.iso:
################---- 80.3%bad status code 504
################---- 80.3% 0.0 kBps aborted

failed to retrieve from systemrescuecd-x86-3.7.1-beta006.iso
Aborting, download available in systemrescuecd-x86-3.7.1-beta006.iso.part
verifying download...
--- snap

zsync uses proxy settings to retrive http://beta.sysresccd.org/systemrescuecd-x86-3.7.1-beta006.iso.zsync, but ignores them while retriving systemrescuecd-3.7.1-beta006.iso. For systemrescuecd-x86-3.7.1-beta006.iso.zsync there is a log entry on the proxy, while for systemrescuecd-3.7.1-beta006.iso is not.
Same result with sniffing: communication is through the proxy for systemrescuecd-x86-3.7.1-beta006.iso.zsync, while zsync tries to get systemrescuecd-3.7.1-beta006.iso without using the proxy. In tune zsync fails downloading the iso.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: zsync 0.6.1-3ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-49.75-generic 3.2.46
Uname: Linux 3.2.0-49-generic x86_64
ApportVersion: 2.0.1-0ubuntu17.3
Architecture: amd64
Date: Thu Jul 11 12:07:02 2013
InstallationMedia: Ubuntu-Server 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120817.3)
MarkForUpload: True
ProcEnviron:
 TERM=screen-bce
 PATH=(custom, no user)
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: zsync
UpgradeStatus: No upgrade log present (probably fresh install)