zsync does not parse http_proxy properly

[Muelli]

I try to use zsync with a corporate proxy.

env https_proxy='http://foo:<email address hidden>:8080/' http_proxy='http://foo:<email address hidden>:8080/' zsync http://cdimage.ubuntu.com/daily-live/current/xenial-desktop-amd64.iso.zsync
foo: Success
failed on url http://cdimage.ubuntu.com/daily-live/current/xenial-desktop-amd64.iso.zsync
could not read control file from URL http://cdimage.ubuntu.com/daily-live/current/xenial-desktop-amd64.iso.zsync

I expected it to work as wget and pretty much every other programme works.

ProblemType: Bug
DistroRelease: Ubuntu 15.04
Package: zsync 0.6.2-1ubuntu1
ProcVersionSignature: Ubuntu 3.19.0-31.36-generic 3.19.8-ckt7
Uname: Linux 3.19.0-31-generic x86_64
ApportVersion: 2.17.2-0ubuntu1.7
Architecture: amd64
CurrentDesktop: GNOME
Date: Wed Dec 2 10:24:22 2015
Dependencies:
 gcc-5-base 5.2.1-15ubuntu2 [origin: unknown]
 libc6 2.21-0ubuntu4
 libgcc1 1:5.2.1-15ubuntu2 [origin: unknown]
 multiarch-support 2.21-0ubuntu4
SourcePackage: zsync
UpgradeStatus: No upgrade log present (probably fresh install)