proto3 JSON: Serialize integer floats and doubles as integers (#695)

This is not required by the spec, but it seems to be more consistent
with how other libraries in other languages generate JSON.

Internally this change was made in cl/405998525.

Author: osa1

protobuf/lib/src/protobuf/proto3_json.dart

@@ -67,6 +67,9 @@ Object? _writeToProto3Json(_FieldSet fs, TypeRegistry typeRegistry) {
           if (value.isInfinite) {
             return value.isNegative ? negativeInfinity : infinity;
           }
+          if (fieldValue.toInt() == fieldValue) {
+            return fieldValue.toInt();
+          }
           return value;
         case PbFieldType._UINT64_BIT:
           return (fieldValue as Int64).toStringUnsigned();

protoc_plugin/test/proto3_json_test.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'dart:convert' show jsonEncode;
 import 'dart:core' hide Duration;
 
 import 'package:fixnum/fixnum.dart';
@@ -1253,6 +1254,7 @@ void main() {
       var proto = TestAllTypes()..optionalDouble = 5.0;
       expect(TestAllTypes()..mergeFromProto3Json(json), proto);
       expect(proto.toProto3Json(), json);
+      expect(jsonEncode(proto.toProto3Json()), '{"optionalDouble":5}');
     });
 
     test('Infinity', () {