Tuple struct length does not account for skip_serializing_if

[dtolnay]

#[derive(Serialize)]
struct S {
    a: usize,
    #[serde(skip_serializing_if = "f")]
    b: usize,
}

#[derive(Serialize)]
struct T(usize, #[serde(skip_serializing_if = "f")] usize);

Struct S invokes serialize_struct with a length of 1 or 2 depending on whether the field is skipped. Tuple struct T invokes serialize_tuple_struct with a length of 2 even if the field is later skipped.

[dtolnay]

Fixed in 67777eb.