In https://tools.ietf.org/html/rfc3442:
The DHCP protocol as defined in RFC 2131 [3] and the options defined in RFC 2132 [4] only provide a mechanism for installing a default route or installing a table of classful routes. Classful routes are routes whose subnet mask is implicit in the subnet number - see section 3.2 of STD 5, RFC 791 [1] for details on classful routing.
So option rfc3442-static-route sould add the default route too.