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/file/tree/elem.go

@@ -91,7 +91,7 @@ func (e *Elem) Delete(rr dns.RR) (empty bool) {
 	return
 }
 
-// Less is a tree helper function that calles middleware.Less.
+// Less is a tree helper function that calls middleware.Less.
 func Less(a *Elem, name string) int { return middleware.Less(name, a.Name()) }
 
 // Assuming the same type and name this will check if the rdata is equal as well.