I just tried this in Shiretoko (4.0b2pre) and it’s fixed there, i.e. the plus sign is preserved. So you probably deduced the right culprit. :)

Steps to reproduce:
1. Go to https://launchpad.net/~mozillateam/+archive/firefox-stable
2. Choose Send link…

Expected outcome:
To have ”https://launchpad.net/~mozillateam/+archive/firefox-stable” as the newly opened email message body.

Actual outcome:
Message body reads ”https://launchpad.net/~mozillateam/ archive/firefox-stable”.