Usernames with uppercase characters cause duplicate assignee avatars to show in Agile Sprint Health Gadget

Users are appearing twice in the Agile Sprint Health Gadget.

Steps to reproduce

1. Add the Agile Sprint Health gadget to a JIRA dashboard, edit it and ensure the "Show assignees" box is ticked.
2. Create a user account whose username has capital letters in it ("AliceUppercase" for example).
3. Assign that user an issue so that their avatar will appear in the Agile Sprint Health gadget.
4. Refresh the gadget.

There'll be two avatars for the one user that has capitals in the username. When mousing-over the two avatars, one will link to "https://<instance>/secure/IssueNavigator.jspa?reset=true&jqlQuery=assignee=aliceuppercase%20AND%20Sprint=<number>" and the other will link to "https://<instance>/secure/IssueNavigator.jspa?reset=true&jqlQuery=assignee=AliceUppercase%20AND%20Sprint=<number>".