add test on Files guard for service filtering

actix-files/tests/fixtures/guards/first/index.txt

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+first

actix-files/tests/fixtures/guards/second/index.txt

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+second

actix-files/tests/guard.rs

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+use actix_files::Files;
+use actix_web::{
+    guard::Host,
+    http::StatusCode,
+    test::{self, TestRequest},
+    App,
+};
+use bytes::Bytes;
+
+#[actix_rt::test]
+async fn test_guard_filter() {
+    let srv = test::init_service(
+        App::new()
+            .service(Files::new("/", "./tests/fixtures/guards/first").guard(Host("first.com")))
+            .service(
+                Files::new("/", "./tests/fixtures/guards/second").guard(Host("second.com")),
+            ),
+    )
+    .await;
+
+    let req = TestRequest::with_uri("/index.txt")
+        .append_header(("Host", "first.com"))
+        .to_request();
+    let res = test::call_service(&srv, req).await;
+
+    assert_eq!(res.status(), StatusCode::OK);
+    assert_eq!(test::read_body(res).await, Bytes::from("first\n"));
+
+    let req = TestRequest::with_uri("/index.txt")
+        .append_header(("Host", "second.com"))
+        .to_request();
+    let res = test::call_service(&srv, req).await;
+
+    assert_eq!(res.status(), StatusCode::OK);
+    assert_eq!(test::read_body(res).await, Bytes::from("second\n"));
+}