Bug 1933239 - Update HTTP Accept header when fetching a JSON module. r=dom-core,necko-reviewers,frontend-codestyle-reviewers,mossop,valentin,sefeng

According to https://fetch.spec.whatwg.org/#fetching

If the request's destination is "json", the Accept value in HTTP header
should be set to `application/json,*/*;q=0.5`.

Differential Revision: https://phabricator.services.mozilla.com/D235837

dom/script/ScriptLoader.cpp

@@ -885,10 +885,8 @@ nsresult ScriptLoader::PrepareHttpRequestAndInitiatorType(
   nsresult rv = NS_OK;
 
   if (httpChannel) {
-    // HTTP content negotation has little value in this context.
-    nsAutoCString acceptTypes("*/*");
-    rv = httpChannel->SetRequestHeader("Accept"_ns, acceptTypes, false);
-    MOZ_ASSERT(NS_SUCCEEDED(rv));
+    // The 'Accept' HTTP header should be set in
+    // nsHttpHandler::AddStandardRequestHeaders.
 
     nsCOMPtr<nsIReferrerInfo> referrerInfo =
         new ReferrerInfo(aRequest->mReferrer, aRequest->ReferrerPolicy());