Fix crash when getting unknown RRs in req.Extra

@40000000512e7adb26a22cc4 panic: interface conversion: dns.RR is *dns.RFC3597, not *dns.OPT
@40000000512e7adb26a2387c
@40000000512e7adb26a23c64 goroutine 8248 [running]:
@40000000512e7adb26a27ae4 main.serve(0x189b4900, 0x1931c690, 0x194c2280, 0x18ca8420, 0x18a4efd4, ...)
@40000000512e7adb26a2c904       /home/ask/go/src/github.com/abh/geodns/serve.go:41 +0x384
@40000000512e7adb26a30784 main._func_005(0x18b09dc8, 0x809f088, 0x189b4900, 0x1931c690, 0x194c2280, ...)
@40000000512e7adb26a3980c       /home/ask/go/src/github.com/abh/geodns/serve.go:167 +0x3e
@40000000512e7adb26a3bb34 github.com/miekg/dns.HandlerFunc.ServeDNS(0x18a4efc0, 0x189b4900, 0x1931c690, 0x194c2280, 0x189b3020, ...)