update/set with Optional fields can lead to an SQL error

[scalexm]

How to reproduce the bug:

#[changeset_for(foo)]
struct Foo {
    field1: i64,
    field2: Option<i16>,
    field3: i16,
}

/* ... */

update(
    foo::table.filter(foo::id.eq(&1))
).set(&Foo {
    field1: 0,
    field2: None,
    field3: 0,
}).execute(&conn).unwrap();

This yields the following error:
ERROR: syntax error at or near "field3"
LINE 1: UPDATE "foo" SET "field1" = $1"field3" = $2

It seems that there is a comma missing in the SQL output with patterns like (Something, None, ..., None, Something)