Update github.com/miekg/dns to pick up fix for CVE-2019-19794.

I noticed this was out of date using 'snyk test' on the main repository.

vendor/github.com/miekg/dns/serve_mux.go

@@ -1,7 +1,6 @@
 package dns
 
 import (
-	"strings"
 	"sync"
 )
 
@@ -36,33 +35,9 @@ func (mux *ServeMux) match(q string, t uint16) Handler {
 		return nil
 	}
 
+	q = CanonicalName(q)
+
 	var handler Handler
-
-	// TODO(tmthrgd): Once https://go-review.googlesource.com/c/go/+/137575
-	// lands in a go release, replace the following with strings.ToLower.
-	var sb strings.Builder
-	for i := 0; i < len(q); i++ {
-		c := q[i]
-		if !(c >= 'A' && c <= 'Z') {
-			continue
-		}
-
-		sb.Grow(len(q))
-		sb.WriteString(q[:i])
-
-		for ; i < len(q); i++ {
-			c := q[i]
-			if c >= 'A' && c <= 'Z' {
-				c += 'a' - 'A'
-			}
-
-			sb.WriteByte(c)
-		}
-
-		q = sb.String()
-		break
-	}
-
 	for off, end := 0, false; !end; off, end = NextLabel(q, off) {
 		if h, ok := mux.z[q[off:]]; ok {
 			if t != TypeDS {
@@ -90,7 +65,7 @@ func (mux *ServeMux) Handle(pattern string, handler Handler) {
 	if mux.z == nil {
 		mux.z = make(map[string]Handler)
 	}
-	mux.z[Fqdn(pattern)] = handler
+	mux.z[CanonicalName(pattern)] = handler
 	mux.m.Unlock()
 }
 
@@ -105,7 +80,7 @@ func (mux *ServeMux) HandleRemove(pattern string) {
 		panic("dns: invalid pattern " + pattern)
 	}
 	mux.m.Lock()
-	delete(mux.z, Fqdn(pattern))
+	delete(mux.z, CanonicalName(pattern))
 	mux.m.Unlock()
 }
 
@@ -116,7 +91,7 @@ func (mux *ServeMux) HandleRemove(pattern string) {
 // are redirected to the parent zone (if that is also registered),
 // otherwise the child gets the query.
 //
-// If no handler is found, or there is no question, a standard SERVFAIL
+// If no handler is found, or there is no question, a standard REFUSED
 // message is returned
 func (mux *ServeMux) ServeDNS(w ResponseWriter, req *Msg) {
 	var h Handler
@@ -127,7 +102,7 @@ func (mux *ServeMux) ServeDNS(w ResponseWriter, req *Msg) {
 	if h != nil {
 		h.ServeDNS(w, req)
 	} else {
-		HandleFailed(w, req)
+		handleRefused(w, req)
 	}
 }