request-base: use `headers` insteadof `header` in `toJSON`
request-base: use `headers` insteadof `header` in `toJSON`

visionmedia/superagent,focusaurus/superagent,fredigato/superagent,visionmedia/superagent,fredigato/superagent,focusaurus/superagent,craigspaeth/superagent,visionmedia/superagent,craigspaeth/superagent