Github is going to make it mandatory: http://developer.github.com/changes/2013-01-31-user-agent-will-soon-be-mandatory/
Although 'default same user-agent for all HTTPClient agent' would not help Github for identifying users, it's better to have User-Agent for helping server admins to identify at the end.
Add User-Agent header by default. Closes #144.
You can remove User-Agent header by setting nil to