Support go 1.15 (#434)

Fixes #428

marshal_test.go

@@ -2194,10 +2194,7 @@ func TestUnmarshalBadDuration(t *testing.T) {
 	result := testBadDuration{}
 	err := NewDecoder(buf).Decode(&result)
 	if err == nil {
-		t.Fatal()
-	}
-	if err.Error() != "(1, 1): Can't convert 1z(string) to time.Duration. time: unknown unit z in duration 1z" {
-		t.Fatalf("unexpected error: %s", err)
+		t.Fatal("expected bad duration error")
 	}
 }