Set a default user agent

I have a use-case where the GitHub API requires a `user-agent` header. Any `user-agent` will do. This patch adds a `node-superagent/{version}` header by default, which can still be overridden by calling `req.set('user-agent', value)`

This allows me to run the same code in both the client (which already sets a user-agent, the browser's), and the server, without some extra hackery to figure out whether I need to set a `user-agent` for Node usage.