update-manager does not follows system proxy setting changes

[Roberto Resoli]

Ho to reproduce:

1) Set a dummy non-existent proxy into System Settings -> Network applet, Apply system-wide.
2) Open update-manager and verify that update informations are not downloaded because of non-existent proxy. Close update-manager.
3) Reset to "none" the proxy settings into System Settings -> Network applet, Apply system-wide.
4 Reopen update-manager, verify that that it still try to connect to the repositories through proxy.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.11
ProcVersionSignature: Ubuntu 3.2.0-32.51-generic 3.2.30
Uname: Linux 3.2.0-32-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu14
Architecture: amd64
Date: Tue Oct 30 14:41:03 2012
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1351603757
 com.ubuntu.update-manager show-details true
 com.ubuntu.update-manager window-height 876
 com.ubuntu.update-manager window-width 988
InstallationMedia: Ubuntu 10.04 "Lucid Lynx" - Beta amd64 (20100317.1)
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=it:en
 PATH=(custom, no user)
 LANG=it_IT.UTF-8
 LC_MESSAGES=it_IT.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
UpgradeStatus: Upgraded to precise on 2012-05-01 (182 days ago)