use GetTime when validating "iat" claim

jwt/jwt.go

diff --git a/jwt/jwt.go b/jwt/jwt.go
index d29c43a..e747bd9 100644
--- a/jwt/jwt.go
+++ b/jwt/jwt.go
@@ -59,9 +59,10 @@
 		j.Claims().Get("sub") != sub {
 		return ErrInvalidSUBClaim
 	}
-	if iat, ok := v.Expected.IssuedAt(); ok &&
-		j.Claims().Get("iat") != iat {
-		return ErrInvalidIATClaim
+	if iat, ok := v.Expected.IssuedAt(); ok {
+		if t, ok := j.Claims().GetTime("iat"); t != iat && ok {
+			return ErrInvalidIATClaim
+		}
 	}
 	if jti, ok := v.Expected.JWTID(); ok &&
 		j.Claims().Get("jti") != jti {