forwarding fails for certain record types

[velartrill]

shaman appears to only forward requests for certain types of DNS record on to the fallback server specified with the -f flag. a cursory test with dig showed that A, AAAA, and CNAME requests are correctly forwarded, while MX and SRV requests are not.

this should be reproducible by launching shaman -f 1.1.1.1 and then comparing the output of dig @1.1.1.1 gmail.com MX with dig @127.0.0.1 gmail.com MX