Fix section.transparent serialization

[wold5]

Fixes passing section.transparent for e8b04bb and #1840

I cobbled a test together from the examples, but suspect it only checks for a section having transparent set internally, and not a template having access to {{ section.transparent }}... Since both rely on meta.transparent anyway, it's perhaps best to leave it at this.

If you have any remarks, let me know.

#[test]
fn check_transparancy() {
    let (mut site, _tmp_dir, _public) = build_site("test_site");
    site.load().unwrap();

    let mut path = env::current_dir().unwrap().parent().unwrap().parent().unwrap().to_path_buf();
    path.push("test_site");
    let posts_path = path.join("content").join("posts");
    let transparent_section_path = posts_path.join("2018");

    let library = site.library.read().unwrap();
    let section = library.sections.get(&transparent_section_path.join("_index.md")).unwrap();   
    assert_eq!(section.meta.transparent, true);
}

[Keats]

Thanks