Case sensitive email address and inconsistent custom avatar

Custom avatar show different if user email address not literally case-matched

step to reproduce,

(on host1)
    $ git pull --rebase
    $ git config user.name "nickname"
    $ git config user.email "nickname@example.com"
    $ touch test1 && git commit -a -m 'test1'
    $ git push

(on host2)
    $ git pull --rebase
    $ git config user.name "nickname"
    $ git config user.email "NickName@example.com"
    $ touch test2 && git commit -a -m 'test2'
    $ git push