Skip headers for Not Modified fs responses (#205)

According to RFC 7232, 304 Not Modified responses should not generate
representation metadata other than the following fields: Cache-Control,
Content-Location, Date, ETag, Expires, and Vary.

This avoids a warning from hyper when logging is enabled:
"content-length value found, but empty body provided". While it is
allowed to provide Content-Length for Not Modified responses it is
unnecessary.

src/filters/fs.rs

@@ -296,7 +296,7 @@ fn file_conditional(
         let mut len = meta.len();
         let modified = meta.modified().ok().map(LastModified::from);
 
-        let mut resp = match conditionals.check(modified) {
+        let resp = match conditionals.check(modified) {
             Cond::NoBody(resp) => resp,
             Cond::WithBody(range) => {
                 bytes_range(range, len)
@@ -317,6 +317,16 @@ fn file_conditional(
                             len = sub_len;
                         }
 
+                        let mime = mime_guess::guess_mime_type(path.as_ref());
+
+                        resp.headers_mut().typed_insert(ContentLength(len));
+                        resp.headers_mut().typed_insert(ContentType::from(mime));
+                        resp.headers_mut().typed_insert(AcceptRanges::bytes());
+
+                        if let Some(last_modified) = modified {
+                            resp.headers_mut().typed_insert(last_modified);
+                        }
+
                         resp
                     })
                     .unwrap_or_else(|BadRange| {
@@ -330,18 +340,6 @@ fn file_conditional(
             }
         };
 
-        if resp.status() != StatusCode::RANGE_NOT_SATISFIABLE {
-            let mime = mime_guess::guess_mime_type(path.as_ref());
-
-            resp.headers_mut().typed_insert(ContentLength(len));
-            resp.headers_mut().typed_insert(ContentType::from(mime));
-            resp.headers_mut().typed_insert(AcceptRanges::bytes());
-
-            if let Some(last_modified) = modified {
-                resp.headers_mut().typed_insert(last_modified);
-            }
-        }
-
         File { resp }
     })
 }

tests/fs.rs

@@ -118,13 +118,16 @@ fn not_modified() {
     let file = warp::fs::file("README.md");
 
     let req = warp::test::request();
+    let body = fs::read("README.md").unwrap();
     let res1 = req.reply(&file);
     assert_eq!(res1.status(), 200);
+    assert_eq!(res1.headers()["content-length"], body.len().to_string());
 
     // if-modified-since
     let res = warp::test::request()
         .header("if-modified-since", &res1.headers()["last-modified"])
         .reply(&file);
+    assert_eq!(res.headers().get("content-length"), None);
     assert_eq!(res.status(), 304);
     assert_eq!(res.body(), "");
 
@@ -133,7 +136,8 @@ fn not_modified() {
         .header("if-modified-since", "Sun, 07 Nov 1994 01:00:00 GMT")
         .reply(&file);
     assert_eq!(res.status(), 200);
-    assert_eq!(res.body(), &*fs::read("README.md").unwrap());
+    assert_eq!(res.body(), &body);
+    assert_eq!(res1.headers()["content-length"], body.len().to_string());
 }
 
 #[test]