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_test.go

@@ -2,6 +2,7 @@ package middleware
 
 import (
 	"sort"
+	"strings"
 	"testing"
 )
 
@@ -53,6 +54,14 @@ func TestLess(t *testing.T) {
 
 Tests:
 	for j, test := range tests {
+		// Need to lowercase these example as the Less function does lowercase for us anymore.
+		for i, b := range test.in {
+			test.in[i] = strings.ToLower(b)
+		}
+		for i, b := range test.out {
+			test.out[i] = strings.ToLower(b)
+		}
+
 		sort.Sort(set(test.in))
 		for i := 0; i < len(test.in); i++ {
 			if test.in[i] != test.out[i] {