lib/response.js
function (field, value) {
    if (arguments.length === 2)
      this.headers[field] = value;
    else {
      for (var prop in field)
        this.headers[prop] = field[prop];
    }
    return this;
  }
function (field, value) {
    if (arguments.length === 2)
      this.headers[field] = value;
    else {
      for (var prop in field)
        this.headers[prop] = field[prop];
    }
    return this;
  }
Sets `field` header to `value`, or accepts an object and applies
those key value pairs to the Response object's headers.

@param {String|Object} field
@param {String} value
@return {Response}