maas doesn't seem to like authenticating proxy URLs

[Muelli]

I tried to setup MAAS behind an authenticating proxy.
When trying to enter "http://foo:bar@proxy/" in the Settings -> Network Configuration -> Proxy for APT and HTTP/HTTPS, I get "Enter a valid URL." as an error.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: maas (not installed)
Uname: Linux 4.6.0-040600-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: GNOME
Date: Fri May 20 17:35:38 2016
InstallationDate: Installed on 2015-12-02 (170 days ago)
InstallationMedia: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Alpha amd64 (20151027)
SourcePackage: maas
UpgradeStatus: No upgrade log present (probably fresh install)