function del(url, fn){
  var req = request('DELETE', url);
  if (fn) req.end(fn);
  return req;
}
Get case-insensitive header `field` value.
This is a deprecated internal API. Use `.get(field)` instead.

(getHeader is no longer used internally by the superagent code base)

@param {String} field
@return {String}
@api private
@deprecated