apt-add-repository cannot add http: repository URL with custom series

I ran into this issue working with a private apt repository at Canonical, in the context of adding a configured repository from a Juju charm install hook, but it seems like a general issue which may affect users of apt-add-repository.

I added a custom repo with:

apt-add-repository "http://archive.admin.canonical.com/ubuntu precise-cat-haproxy main"

This appended the following to /etc/apt/sources.list:

deb http://archive.admin.canonical.com/ubuntu precise main precise-cat-haproxy
deb-src http://archive.admin.canonical.com/ubuntu precise main precise-cat-haproxy

But what I wanted was:

deb http://archive.admin.canonical.com/ubuntu precise-cat-haproxy main
deb-src http://archive.admin.canonical.com/ubuntu precise-cat-haproxy main

I could see this affecting other users who maintain their own apt repositories and want to specify their own series.

Environment details:

root@juju-canonistack-machine-17:~# lsb_release -rd
Description: Ubuntu 12.04.4 LTS
Release: 12.04

ii python-apt 0.8.3ubuntu7.1 Python interface to libapt-pkg
ii python-apt-common 0.8.3ubuntu7.1 Python interface to libapt-pkg (locales)