fix(ext/flash): Avoid sending Content-Length when status code is 204 (#15901)

Currently Content-Length is sent when the status code is 204. However,
according to the spec, this should not be sent.
Modify the if statement below to prevent the Content-Length from being
sent.

cli/tests/unit/flash_test.ts

@@ -1888,6 +1888,33 @@ Deno.test(
   },
 );
 
+Deno.test(
+  { permissions: { net: true } },
+  async function httpServer204ResponseDoesntSendContentLength() {
+    const listeningPromise = deferred();
+    const ac = new AbortController();
+    const server = Deno.serve({
+      handler: (_request) => new Response(null, { status: 204 }),
+      port: 4501,
+      signal: ac.signal,
+      onListen: onListen(listeningPromise),
+      onError: createOnErrorCb(ac),
+    });
+
+    try {
+      await listeningPromise;
+      const resp = await fetch("http://127.0.0.1:4501/", {
+        method: "GET",
+        headers: { "connection": "close" },
+      });
+      assertEquals(resp.headers.get("Content-Length"), null);
+    } finally {
+      ac.abort();
+      await server;
+    }
+  },
+);
+
 Deno.test(
   { permissions: { net: true } },
   async function httpServer304ResponseDoesntSendBody() {

ext/flash/01_http.js

@@ -152,7 +152,8 @@
     }
 
     // MUST NOT send Content-Length or Transfer-Encoding if status code is 1xx or 204.
-    if (status == 204 && status <= 100) {
+    if (status === 204 || status < 200) {
+      str += "\r\n";
       return str;
     }