middleware/httpproxy: add debug queries (#446)

* middleware/httproxy: implement debug queries

Not too useful at the moment, but o-o.debug queries are supported
and return the Comment from dns.google.com.

Note that this is not always set.

* improve documentation

* Testing cleanups

middleware/pkg/debug/debug.go

@@ -0,0 +1,20 @@
+package debug
+
+import "strings"
+
+const Name = "o-o.debug."
+
+// IsDebug checks if name is a debugging name, i.e. starts with o-o.debug.
+// it returns the empty string if it is not a debug message, otherwise it will return the
+// name with o-o.debug. stripped off. Must be called with name lowercased.
+func IsDebug(name string) string {
+	if len(name) == len(Name) {
+		return ""
+	}
+	name = strings.ToLower(name)
+	debug := strings.HasPrefix(name, Name)
+	if !debug {
+		return ""
+	}
+	return name[len(Name):]
+}