Function fn_guard 

pub fn fn_guard<F>(f: F) -> impl Guard
where
    F: Fn(&RequestHead) -> bool,

Create guard object for supplied function.

use ntex::web::{self, guard, App, HttpResponse};

fn main() {
    App::default().service(web::resource("/index.html").route(
        web::route()
            .guard(
                guard::fn_guard(
                    |req| req.headers()
                             .contains_key("content-type")))
            .to(async || { HttpResponse::MethodNotAllowed() }))
    );
}