derp: remove JSON struct tags in comments

They don't work in comments.

Added a test too to show that there's no change in behavior.
(It does case insensitive matching on parse anyway)

derp/derp_server.go

@@ -712,7 +712,7 @@ func (s *Server) sendServerKey(bw *bufio.Writer) error {
 }
 
 type serverInfo struct {
-	Version int // `json:"version,omitempty"`
+	Version int `json:"version,omitempty"`
 }
 
 func (s *Server) sendServerInfo(bw *bufio.Writer, clientKey key.Public) error {