Bug#622701: HTTP error 403 when run on apt-cacher-ng client configured with proxy

[Dominique Brazziel]

	I just found out a case where the changelog viewer fails to download
the changelog when '/etc/apt/apt.conf.d/NNproxy' is set, as can be seen
in the apt-cacher-ng server access log, i.e.:

403 Forbidden file type or location:
http://packages.debian.org/changelogs/pool/main/b/bluez/bluez_4.99-1/changelog.txt

When proxy support was added to update-manager (see 
sourcetree/UpdateManager/Frontend/Gtk/utils.py), the default
url handler uses the proxy for downloading changelogs as well as the
package debs.  See also -

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=664542