Can't install packages from apt: links behind HTTP Proxy

[Jiří Janoušek]

Ubuntu 17.04 amd64, apturl 0.5.2ubuntu12

Steps to reproduce:

1. Set up HTTP proxy in network preferences
2. Click on apt link in Firefox, e.g. apt:flatpak
3. Apt url dialog opens, click Install.
4. Type password and confirm installation.
5. An error dialog opens:

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Failed to download package files

Check your Internet connection.

Details:

Failed to fetch http://cz.archive.ubuntu.com/ubuntu/pool/universe/b/bubblewrap/bubblewrap_0.1.7-1_amd64.deb Could not connect to cz.archive.ubuntu.com:80 (217.31.202.63). - connect (111: Connection refused)
Failed to fetch http://cz.archive.ubuntu.com/ubuntu/pool/universe/o/ostree/libostree-1-1_2016.15-3ubuntu3_amd64.deb Unable to connect to cz.archive.ubuntu.com:http:
Failed to fetch http://cz.archive.ubuntu.com/ubuntu/pool/universe/f/flatpak/flatpak_0.8.5-1_amd64.deb Unable to connect to cz.archive.ubuntu.com:http:
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Note that `sudo apt install flatpak` does work and my system is able to reach cz.archive.ubuntu.com:80 via the http proxy.

ProblemType: Bug
DistroRelease: Ubuntu 17.04
Package: apturl 0.5.2ubuntu12
ProcVersionSignature: Ubuntu 4.10.0-19.21-generic 4.10.8
Uname: Linux 4.10.0-19-generic x86_64
ApportVersion: 2.20.4-0ubuntu4
Architecture: amd64
CurrentDesktop: Unity:Unity7
Date: Mon Apr 17 13:06:38 2017
InstallationDate: Installed on 2017-04-09 (8 days ago)
InstallationMedia: Ubuntu 17.04 "Zesty Zapus" - Beta amd64 (20170321)
SourcePackage: apturl
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.