feat: confirm a weak but matching ETag (#3485)

When handling compression at the proxy server level, the client receives a weak ETag.
Weak ETags are prefixed with `W/`, e.g. `W/"2.2.0"`.
Upon cache validation we should take care of these too.

Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/ETag

lib/index.ts

@@ -437,10 +437,11 @@ export class Server extends EventEmitter {
     // Per the standard, ETags must be quoted:
     // https://tools.ietf.org/html/rfc7232#section-2.3
     const expectedEtag = '"' + clientVersion + '"';
+    const weakEtag = "W/" + expectedEtag;
 
     const etag = req.headers["if-none-match"];
     if (etag) {
-      if (expectedEtag == etag) {
+      if (expectedEtag === etag || weakEtag === etag) {
         debug("serve client %s 304", type);
         res.writeHead(304);
         res.end();

test/socket.io.ts

@@ -128,6 +128,19 @@ describe("socket.io", () => {
           });
       });
 
+      it("should handle 304", (done) => {
+        const srv = createServer();
+        new Server(srv);
+        request(srv)
+          .get("/socket.io/socket.io.js")
+          .set("If-None-Match", 'W/"' + clientVersion + '"')
+          .end((err, res) => {
+            if (err) return done(err);
+            expect(res.statusCode).to.be(304);
+            done();
+          });
+      });
+
       it("should not serve static files", (done) => {
         const srv = createServer();
         new Server(srv, { serveClient: false });