encoding/jsonpb: fix encoding of max/min Duration and max Timestamp

Bad off-by-one error:(

Change-Id: I47dfaa8529fcdb2d53a4fc67c1f046d2152fe8fe
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/173837
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/encoding/jsonpb/encode_test.go b/encoding/jsonpb/encode_test.go
index 0c12861..b0e3973 100644
--- a/encoding/jsonpb/encode_test.go
+++ b/encoding/jsonpb/encode_test.go
@@ -1422,6 +1422,14 @@
 		input: &knownpb.Duration{Seconds: -123, Nanos: -450},
 		want:  `"-123.000000450s"`,
 	}, {
+		desc:  "Duration max value",
+		input: &knownpb.Duration{Seconds: 315576000000, Nanos: 999999999},
+		want:  `"315576000000.999999999s"`,
+	}, {
+		desc:  "Duration min value",
+		input: &knownpb.Duration{Seconds: -315576000000, Nanos: -999999999},
+		want:  `"-315576000000.999999999s"`,
+	}, {
 		desc:    "Duration with +secs -nanos",
 		input:   &knownpb.Duration{Seconds: 1, Nanos: -1},
 		wantErr: true,
@@ -1466,6 +1474,14 @@
 		input: &knownpb.Timestamp{Nanos: 1e7},
 		want:  `"1970-01-01T00:00:00.010Z"`,
 	}, {
+		desc:  "Timestamp max value",
+		input: &knownpb.Timestamp{Seconds: 253402300799, Nanos: 999999999},
+		want:  `"9999-12-31T23:59:59.999999999Z"`,
+	}, {
+		desc:  "Timestamp min value",
+		input: &knownpb.Timestamp{Seconds: -62135596800},
+		want:  `"0001-01-01T00:00:00Z"`,
+	}, {
 		desc:    "Timestamp with +secs out of range",
 		input:   &knownpb.Timestamp{Seconds: 253402300800},
 		wantErr: true,