fix(date test): Fail with invalid date

Constructs a datestring with RFC3339 formatting using the parsed date
values, then attempts to call `time.Parse()` on it, which returns a
non-nil error if datetime string was invalid or `time`'s parsing of
the string failed.

Resolves: part of pelletier#613

decode.go

@@ -99,6 +99,13 @@ func parseDateTime(b []byte) (time.Time, error) {
 		return time.Time{}, newDecodeError(b, "extra bytes at the end of the timezone")
 	}
 
+	dateString := fmt.Sprintf("%04d-%02d-%02dT%02d:%02d:%02dZ",
+		dt.Year, dt.Month, dt.Day, dt.Hour, dt.Minute, dt.Second)
+	_, err = time.Parse(time.RFC3339, dateString)
+	if err != nil {
+		return time.Time{}, newDecodeError(b, "invalid date-time")
+	}
+
 	t := time.Date(
 		dt.Year,
 		time.Month(dt.Month),