refactor: add charset when serving the bundle files

Reference: https://developer.mozilla.org/fr/docs/Web/HTTP/Headers/Content-Type

Related: #4589

Author: darrachequesne

lib/index.ts

@@ -488,7 +488,7 @@ export class Server<
         res.writeHeader("cache-control", "public, max-age=0");
         res.writeHeader(
           "content-type",
-          "application/" + (isMap ? "json" : "javascript")
+          "application/" + (isMap ? "json" : "javascript") + "; charset=utf-8"
         );
         res.writeHeader("etag", expectedEtag);
 
@@ -581,7 +581,7 @@ export class Server<
     res.setHeader("Cache-Control", "public, max-age=0");
     res.setHeader(
       "Content-Type",
-      "application/" + (isMap ? "json" : "javascript")
+      "application/" + (isMap ? "json" : "javascript") + "; charset=utf-8"
     );
     res.setHeader("ETag", expectedEtag);
 

test/server-attachment.ts

@@ -16,7 +16,9 @@ describe("server attachment", () => {
         .buffer(true)
         .end((err, res) => {
           if (err) return done(err);
-          expect(res.headers["content-type"]).to.be("application/javascript");
+          expect(res.headers["content-type"]).to.be(
+            "application/javascript; charset=utf-8"
+          );
           expect(res.headers.etag).to.be('"' + clientVersion + '"');
           expect(res.headers["x-sourcemap"]).to.be(undefined);
           expect(res.text).to.match(/engine\.io/);
@@ -33,7 +35,9 @@ describe("server attachment", () => {
         .buffer(true)
         .end((err, res) => {
           if (err) return done(err);
-          expect(res.headers["content-type"]).to.be("application/json");
+          expect(res.headers["content-type"]).to.be(
+            "application/json; charset=utf-8"
+          );
           expect(res.headers.etag).to.be('"' + clientVersion + '"');
           expect(res.text).to.match(/engine\.io/);
           expect(res.status).to.be(200);

test/uws.ts

@@ -200,7 +200,9 @@ describe("socket.io with uWebSocket.js-based engine", () => {
       .buffer(true)
       .end((err, res) => {
         if (err) return done(err);
-        expect(res.headers["content-type"]).to.be("application/javascript");
+        expect(res.headers["content-type"]).to.be(
+          "application/javascript; charset=utf-8"
+        );
         expect(res.headers.etag).to.be('"' + clientVersion + '"');
         expect(res.headers["x-sourcemap"]).to.be(undefined);
         expect(res.text).to.match(/engine\.io/);