Confirm-Mail doesn't work for "Send Later" command.

Environment:
Thunderbird 17.0.8 + Confirm-Mail 1.2, on Ubuntu

Steps to reproduce:
1. Start to compose a new message.
2. Choose "File" => "Send Later" in the composer window.

Expected result:
The dialog of Confirm-Mail is shown and I can cancel to send the composing message. After the "OK" button is pressed, the message is stored into the "Outbox" folder of the local folder account.

Actual result:
The composing message is stored into the "Outbox" folder of the local folder account, without any confirmation.

The attached patch will solve this issue. Could you merge it to the next release?