MarshalJSON should be defined on T not *T

Otherwise "%v" format would be used by json.Marshal(T).

empty_query_response.go

@@ -25,7 +25,7 @@ func (src *EmptyQueryResponse) Encode(dst []byte) []byte {
 }
 
 // MarshalJSON implements encoding/json.Marshaler.
-func (src *EmptyQueryResponse) MarshalJSON() ([]byte, error) {
+func (src EmptyQueryResponse) MarshalJSON() ([]byte, error) {
 	return json.Marshal(struct {
 		Type string
 	}{