middleware/file: Support delegations (#124)

Return a delegation when seeing one while traversing the tree in
search of an answer.

Put the SOA and NS record in the zone.Apex as these are to be handled
somewhat special.

Lowercase record on insert to make compares easier. This lowercases
all RR that have domain names in their rdata as well.

middleware/canonical.go

@@ -12,6 +12,8 @@ import (
 //
 // See http://bert-hubert.blogspot.co.uk/2015/10/how-to-do-fast-canonical-ordering-of.html
 // for a blog article on this implementation.
+//
+// The values of a and b are *not* lowercased before the comparison!
 func Less(a, b string) int {
 	i := 1
 	aj := len(a)
@@ -22,11 +24,12 @@ func Less(a, b string) int {
 		if oka && okb {
 			return 0
 		}
-		// sadly this []byte will allocate...
+		// sadly this []byte will allocate... TODO(miek): check if this is needed
+		// for a name, otherwise compare the strings.
 		ab := []byte(a[ai:aj])
-		toLowerAndDDD(ab)
 		bb := []byte(b[bi:bj])
-		toLowerAndDDD(bb)
+		doDDD(ab)
+		doDDD(bb)
 
 		res := bytes.Compare(ab, bb)
 		if res != 0 {
@@ -39,13 +42,9 @@ func Less(a, b string) int {
 	return 0
 }
 
-func toLowerAndDDD(b []byte) {
+func doDDD(b []byte) {
 	lb := len(b)
 	for i := 0; i < lb; i++ {
-		if b[i] >= 'A' && b[i] <= 'Z' {
-			b[i] += 32
-			continue
-		}
 		if i+3 < lb && b[i] == '\\' && isDigit(b[i+1]) && isDigit(b[i+2]) && isDigit(b[i+3]) {
 			b[i] = dddToByte(b[i:])
 			for j := i + 1; j < lb-3; j++ {