back (alt) {
    const url = this.ctx.get('Referrer') || alt || '/'
    this.redirect(url)
  }
Perform a special-cased "back" to provide Referrer support.
When Referrer is not present, `alt` or "/" is used.

Examples:

   ctx.back()
   ctx.back('/index.html')

@param {String} [alt]
@api public