serde_test 1.0.20+ fails on Struct len check

[bradleybeddoes]

This appears to be failing because the struct in question makes use of skip_serializing_if = Option::is_none.

Here is an example which passes prior to serde_test 1.0.20

    #[derive(Serialize, Deserialize, PartialEq, Debug)]
    struct A {
        pub x: u8,

        #[serde(default, skip_serializing_if = "Option::is_none")]
        pub y: Option<u8>,
    }

    #[test]
    pub fn test_struct_a() {
        let a = A { x: 1, y: None };

        assert_tokens(
            &a,
            &[
                Token::Struct { name: "A", len: 1 },
                Token::Str("x"),
                Token::U8(1),
                Token::StructEnd,
            ],
        );
    }

The result from a test run is:

... 
panicked at 'expected Token::Struct { name: "A", len: 1 } but deserialization wants Token::Struct { name: "A", len: 2 }'
...

[dtolnay]

Thanks, I reverted the change in eb6bf16a5191df1e736619a8dd2a05749c86b8df and released serde_test 1.0.23.