Custom key combination doesn't work on reassigning with system default shortcut

[pangolin]

I found one bug on Ubuntu 12.04.5 LTS.

How to reproduce:
1. Go to <System Settings> and click on <Keyboard> /<Shortcut> tab.

2. Navigate to <Custom Shortcuts> and then you can add new custom shortcuts following as:
<Ctrl><Alt><Delete> for evince
<Ctrl><Alt>l for gedit

3. In this case, you can see the dialog which prompts the "The shortcut is already used for Lock Screen/ Log out. If you reassign the shortcut to gedit/evince, the Lock Screen/Log out shortcut will be disabled."

4. Once you click on the "reassign" button, you can just try to use custom key combinations such as <Ctrl><Alt><Delete>，<Ctrl><Alt>l in order to run application.

It doesn't work.

Expected result:
Applications should be run upon click of custom key combinations such as <Ctrl><Alt><Delete>，<Ctrl><Alt>l.

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "fix_custom_shortcut_for_system_default.patch" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

[This is an automated message performed by a Launchpad user owned by ~brian-murray, for any issues please contact him.]

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[pangolin (prepangolin-x)]

I've fixed the bug, and see the attachement if you can apply this please.
Hope to review.