EDNS: return error on wrong version. (#95)

Split up the previous changes a bit. This PR only returns the expected
error when the received packet has the wrong EDNS version.

EDNS0 handling in the middleware needs a nicer abstraction, like
ReflectEdns() or something.

middleware/rcode.go

@@ -0,0 +1,14 @@
+package middleware
+
+import (
+	"strconv"
+
+	"github.com/miekg/dns"
+)
+
+func RcodeToString(rcode int) string {
+	if str, ok := dns.RcodeToString[rcode]; ok {
+		return str
+	}
+	return "RCODE" + strconv.Itoa(rcode)
+}