function Request(method, url) {
  var self = this;
  Emitter.call(this);
  this._query = this._query || [];
  this.method = method;
  this.url = url;
  this.header = {};
  this._header = {};
  this.on('end', function(){
    var res = new Response(self);
    if ('HEAD' == method) res.text = null;
    self.callback(null, res);
  });
}
Get case-insensitive header `field` value.

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