GPG_AGENT_INFO not being passed to thunderbird

On Trusty, if I launch thunderbird from the terminal or from the Dash, then thunderbird is able to use enigmail. If I launch via indicator-messages, it is not.

Steps to reproduce:
1. adjust /usr/lib/thunderbird/thunderbird.sh to have:
echo "TEST: $GPG_AGENT_INFO" > /tmp/tbird.gpg

2. Launch from a terminal:
$ rm -f /tmp/tbird.gpg ; thunderbird
$ cat /tmp/tbird.gpg # separate terminal
TEST: /run/user/1000/keyring-bzAKyI/gpg:0:1
(It worked!)

3. Launch from the Dash:
$ rm -f /tmp/tbird.gpg
<launch from Dash>
$ cat /tmp/tbird.gpg # separate terminal
TEST: /run/user/1000/keyring-bzAKyI/gpg:0:1
(It worked!)

4. Launch from indicator-messages:
$ rm -f /tmp/tbird.gpg
<select Thunderbird Mail from indicator-messages>
$ cat /tmp/tbird.gpg # separate terminal
TEST:
(It did not work)

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: indicator-messages 13.10.1+14.04.20131125-0ubuntu1
ProcVersionSignature: Ubuntu 3.12.0-5.13-generic 3.12.2
Uname: Linux 3.12.0-5-generic x86_64
ApportVersion: 2.12.7-0ubuntu1
Architecture: amd64
CurrentDesktop: Unity
Date: Tue Dec 10 07:59:07 2013
InstallationDate: Installed on 2011-06-28 (895 days ago)
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110425.2)
SourcePackage: indicator-messages
UpgradeStatus: Upgraded to trusty on 2013-11-04 (35 days ago)