Fix to set generic userAgent if none configured if using proxy

As per discussion with @satazor in previous issue [https://github.com/bower/bower/issues/698#issuecomment-22215528], made a fix to change the userAgent config from a generic node.js one (ex. `'node/v0.10.13 darwin x64'`) to a generic entry that isn't likely to be filtered by aggressive corporate proxy/firewall setups. In some cases, corporate firewalls will automatically reject non-tunneled http traffic that occurs with blank or unfamiliar `'User-Agent'` strings in the header. This is a tested solution to help those trapped behind proxies that are unfriendly to node `'User-Agent'` strings. 

In this case, I used `curl`, since it minimally identifies the traffic as a non-browser and is well-known and innocuous.