v3.8.0

• Added support for "globally" defined headers and event handlers via superagent.agent(). It now remembers default settings for all its requests.
• Added optional callback to .retry() (Alexander Murphy)
• Unified auth args handling in node/browser (Edmundo Alvarez)
• Fixed error handling in zlib pipes (Kornel)
• Documented that 3xx status codes are errors (Mickey Reiss)