SerJson generates trailing "," when serializing struct with Option

[marot]

Here is a test to demonstrate the issue:

#[derive(SerJson)]
struct TestStruct {
    a: Option<String>,
    b: Option<String>,
}
#[test]
fn test_serialization() {
    let test_struct = TestStruct {
        a: Some("string".to_owned()),
        b: None,
    };
    let test_struct2 = TestStruct {
        a: None,
        b: Some("string".to_owned()),
    };
    let result = test_struct.serialize_json();
    let result2 = test_struct2.serialize_json();

    // This one is correct
    assert_eq!(result2, "{\"b\":\"string\"}");
    // This one has an additional trailing ","
    assert_eq!(result, "{\"a\":\"string\"}");
}

I think the problem is that in derive_ser_json_struct the index of the last item is determined using
let last = struct_.fields.len() - 1; but when options are involved, the last index might be lower.