set status (code) {
    if (this.headerSent) return

    assert(Number.isInteger(code), 'status code must be a number')
    assert(code >= 100 && code <= 999, `invalid status code: ${code}`)
    this._explicitStatus = true
    this.res.statusCode = code
    if (this.req.httpVersionMajor < 2) this.res.statusMessage = statuses.message[code]
    if (this.body && statuses.empty[code]) this.body = null
  }
Set response status code.

@param {Number} code
@api public