v4 preview: *jwt.Time does not always unmarshal back to its initial value.

[dcormier]

This test fails when using v4.0.0-preview1:

func TestJWTTime(t *testing.T) {
	when, _ := time.Parse(time.RFC3339Nano, "2017-07-31T12:59:59.999999999Z")
	expected := jwt.At(when)
	js, _ := json.Marshal(expected)
	var got jwt.Time
	json.Unmarshal(js, &got)

	t.Logf("Initial time.Time:     %s", when.Format(time.RFC3339Nano))
	t.Logf("Expected *jwt.Time:    %s", expected.Format(time.RFC3339Nano))
	t.Logf("JSON:                  %s", js)
	t.Logf("Unmarshaled *jwt.Time: %s", got.UTC().Format(time.RFC3339Nano))

	if expected.Format(time.RFC3339Nano) != got.UTC().Format(time.RFC3339Nano) {
		t.Fatalf("Did not get expected %T after unmarshaling", expected)
	}
}

If you run it (playground), you'll get this failure:

=== RUN   TestJWTTime
    TestJWTTime: prog.go:18: Initial time.Time:     2017-07-31T12:59:59.999999999Z
    TestJWTTime: prog.go:19: Expected *jwt.Time:    2017-07-31T12:59:59.999999Z
    TestJWTTime: prog.go:20: JSON:                  1501505999.999999
    TestJWTTime: prog.go:21: Unmarshaled *jwt.Time: 2017-07-31T12:59:59.999998Z
    TestJWTTime: prog.go:24: Did not get expected *jwt.Time after unmarshaling
--- FAIL: TestJWTTime (0.00s)
FAIL

I ran into this while doing a test where I needed a JWT and found that my parsed JWT didn't match my initial JWT. Tracked it back to this.

[ygj6]

This problem is caused by the deserialization calculation precision.