Can't install apps via clicking on apt url

[ Description ]

Slashes are being appended to URLs passed to apturl, which it isn't prepared to handle.

[ QA ]

Test 1.

1. Open a terminal
2. Run `xdg-open apt://gnome-terminal'

You should see an error message if it is broken, and either a message about gnome-terminal being already installed or an installation prompt from apturl if it is not broken.

Test 2.

1. Click this link: apt://gnome-terminal

As above (assuming your browser is configured to correctly pass apt URLs over to the system's mimetype handler).

[ Regression potential ]

If the fix works, apturl will start working. A potential regression is that apturl is broken in some way that causes damage to systems.

[ Original description ]

Running latest artful.
Visit http://www.videolan.org/vlc/download-ubuntu.html
Click big orange "Available on the software center" button (which links to apt://vlc

Expected outcome:-

GNOME Software which would find vlc in the archive

Actual outcome:-

A dialog box appears
"Invalid URL: 'apt://vlc/' given, exiting
Invalid package name 'vlc/'"

See screenshot.

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: apturl 0.5.2ubuntu12
ProcVersionSignature: Ubuntu 4.13.0-16.19-generic 4.13.4
Uname: Linux 4.13.0-16-generic x86_64
ApportVersion: 2.20.7-0ubuntu3
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Mon Oct 23 10:30:09 2017
InstallationDate: Installed on 2017-08-02 (81 days ago)
InstallationMedia: Ubuntu 17.10 "Artful Aardvark" - Alpha amd64 (20170712)
SourcePackage: apturl
UpgradeStatus: No upgrade log present (probably fresh install)