'=' does not work properly for DataType::Union

[vserret]

This test does not pass

#[test]
    fn test_equalities_union_struct() {
        let s1 = DataType::structured([
            ("a", DataType::float()),
            ("b", DataType::float()),
        ]);
        let s2 = DataType::structured([
            ("a", DataType::boolean()),
            ("b", DataType::integer()),
        ]);
        assert!(s1 != s2);

        let u1 = DataType::union([
            ("table1", s1.clone()),
            ("table2", s2.clone()),
        ]);
        let u2 = DataType::union([
            ("table1", s1.clone()),
            ("table2", s1.clone()),
        ]);
        assert!(u1 != u2)
    }

[vserret]

Fixed in #106