jsonpb: change Marshal to trim timestamp/duration to 0 fractional digits if possible (#490)

According to the proto3 to JSON specification, the JSON representation of a timestamp field:
>>>
Uses RFC 3339, where generated output will always be Z-normalized and uses 0, 3, 6 or 9 fractional digits.
<<<

jsonpb/jsonpb.go

@@ -200,7 +200,7 @@ func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent, typeU
 			// Any is a bit more involved.
 			return m.marshalAny(out, v, indent)
 		case "Duration":
-			// "Generated output always contains 3, 6, or 9 fractional digits,
+			// "Generated output always contains 0, 3, 6, or 9 fractional digits,
 			//  depending on required precision."
 			s, ns := s.Field(0).Int(), s.Field(1).Int()
 			if ns <= -secondInNanos || ns >= secondInNanos {
@@ -215,6 +215,7 @@ func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent, typeU
 			x := fmt.Sprintf("%d.%09d", s, ns)
 			x = strings.TrimSuffix(x, "000")
 			x = strings.TrimSuffix(x, "000")
+			x = strings.TrimSuffix(x, ".000")
 			out.write(`"`)
 			out.write(x)
 			out.write(`s"`)
@@ -225,7 +226,7 @@ func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent, typeU
 			return m.marshalValue(out, &proto.Properties{}, s.Field(0), indent)
 		case "Timestamp":
 			// "RFC 3339, where generated output will always be Z-normalized
-			//  and uses 3, 6 or 9 fractional digits."
+			//  and uses 0, 3, 6 or 9 fractional digits."
 			s, ns := s.Field(0).Int(), s.Field(1).Int()
 			if ns < 0 || ns >= secondInNanos {
 				return fmt.Errorf("ns out of range [0, %v)", secondInNanos)
@@ -235,6 +236,7 @@ func (m *Marshaler) marshalObject(out *errWriter, v proto.Message, indent, typeU
 			x := t.Format("2006-01-02T15:04:05.000000000")
 			x = strings.TrimSuffix(x, "000")
 			x = strings.TrimSuffix(x, "000")
+			x = strings.TrimSuffix(x, ".000")
 			out.write(`"`)
 			out.write(x)
 			out.write(`Z"`)

jsonpb/jsonpb_test.go

@@ -406,7 +406,8 @@ var marshalingTests = []struct {
 	{"Any with message and indent", marshalerAllOptions, anySimple, anySimplePrettyJSON},
 	{"Any with WKT", marshaler, anyWellKnown, anyWellKnownJSON},
 	{"Any with WKT and indent", marshalerAllOptions, anyWellKnown, anyWellKnownPrettyJSON},
-	{"Duration", marshaler, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 3}}, `{"dur":"3.000s"}`},
+	{"Duration", marshaler, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 3}}, `{"dur":"3s"}`},
+	{"Duration", marshaler, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 3, Nanos: 1e6}}, `{"dur":"3.001s"}`},
 	{"Duration beyond float64 precision", marshaler, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 100000000, Nanos: 1}}, `{"dur":"100000000.000000001s"}`},
 	{"negative Duration", marshaler, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: -123, Nanos: -456}}, `{"dur":"-123.000000456s"}`},
 	{"Struct", marshaler, &pb.KnownTypes{St: &stpb.Struct{
@@ -423,6 +424,7 @@ var marshalingTests = []struct {
 		{Kind: &stpb.Value_BoolValue{true}},
 	}}}, `{"lv":["x",null,3,true]}`},
 	{"Timestamp", marshaler, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: 14e8, Nanos: 21e6}}, `{"ts":"2014-05-13T16:53:20.021Z"}`},
+	{"Timestamp", marshaler, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: 14e8, Nanos: 0}}, `{"ts":"2014-05-13T16:53:20Z"}`},
 	{"number Value", marshaler, &pb.KnownTypes{Val: &stpb.Value{Kind: &stpb.Value_NumberValue{1}}}, `{"val":1}`},
 	{"null Value", marshaler, &pb.KnownTypes{Val: &stpb.Value{Kind: &stpb.Value_NullValue{stpb.NullValue_NULL_VALUE}}}, `{"val":null}`},
 	{"string number value", marshaler, &pb.KnownTypes{Val: &stpb.Value{Kind: &stpb.Value_StringValue{"9223372036854775807"}}}, `{"val":"9223372036854775807"}`},
@@ -677,8 +679,10 @@ var unmarshalingTests = []struct {
 	{"camelName input", Unmarshaler{}, `{"oBool":true}`, &pb.Simple{OBool: proto.Bool(true)}},
 
 	{"Duration", Unmarshaler{}, `{"dur":"3.000s"}`, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 3}}},
+	{"Duration", Unmarshaler{}, `{"dur":"4s"}`, &pb.KnownTypes{Dur: &durpb.Duration{Seconds: 4}}},
 	{"null Duration", Unmarshaler{}, `{"dur":null}`, &pb.KnownTypes{Dur: nil}},
 	{"Timestamp", Unmarshaler{}, `{"ts":"2014-05-13T16:53:20.021Z"}`, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: 14e8, Nanos: 21e6}}},
+	{"Timestamp", Unmarshaler{}, `{"ts":"2014-05-13T16:53:20Z"}`, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: 14e8, Nanos: 0}}},
 	{"PreEpochTimestamp", Unmarshaler{}, `{"ts":"1969-12-31T23:59:58.999999995Z"}`, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: -2, Nanos: 999999995}}},
 	{"ZeroTimeTimestamp", Unmarshaler{}, `{"ts":"0001-01-01T00:00:00Z"}`, &pb.KnownTypes{Ts: &tspb.Timestamp{Seconds: -62135596800, Nanos: 0}}},
 	{"null Timestamp", Unmarshaler{}, `{"ts":null}`, &pb.KnownTypes{Ts: nil}},