Return an instance with the specified status code and, optionally, reason phrase.

If no reason phrase is specified, will default recommended reason phrase for
the response's status code.

@see http://tools.ietf.org/html/rfc7231#section-6
@see http://www.iana.org/assignments/http-status-codes/http-status-codes.xhtml

@param integer $code   The 3-digit integer result code to set.
@param string  $reason The reason phrase to use with the
provided status code; if none is provided, will
default to the IANA name.

@return $this
@throws \CodeIgniter\HTTP\Exceptions\HTTPException For invalid status code arguments.