set lastModified (val) {
    if (typeof val === 'string') val = new Date(val)
    this.set('Last-Modified', val.toUTCString())
  }
Set the Last-Modified date using a string or a Date.

    this.response.lastModified = new Date();
    this.response.lastModified = '2013-09-13';

@param {String|Date} val
@api public