Rebuild Response's public methods

[kanarus]

For more consistent names, and, some fangs like HTML layout fang:

struct LayoutFang;
impl FangAction for LayoutFang {
    async fn back<'b>(&'b self, res: &'b mut Response) {
        if res.headers.ContentType().is_some_and(|ct| ct.starts_with("text/html")) {
            let inner = res.drop_content();
            res.set_html(some_layout_fn(inner));
        }
    }
}