use body when Request instance is passed to fetch (fixes #2433) (#2435)

js/fetch.ts

@@ -403,6 +403,11 @@ export async function fetch(
     url = input.url;
     method = input.method;
     headers = input.headers;
+
+    //@ts-ignore
+    if (input._bodySource) {
+      body = new DataView(await input.arrayBuffer());
+    }
   }
 
   // Send Fetch message

js/fetch_test.ts

@@ -99,7 +99,6 @@ testPerm({ net: true }, async function fetchInitStringBody(): Promise<void> {
   assert(response.headers.get("content-type").startsWith("text/plain"));
 });
 
-/* TODO(ry) Re-enable this test.
 testPerm({ net: true }, async function fetchRequestInitStringBody(): Promise<
   void
 > {
@@ -111,9 +110,7 @@ testPerm({ net: true }, async function fetchRequestInitStringBody(): Promise<
   const response = await fetch(req);
   const text = await response.text();
   assertEquals(text, data);
-  assert(response.headers.get("content-type").startsWith("text/plain"));
 });
-*/
 
 testPerm({ net: true }, async function fetchInitTypedArrayBody(): Promise<
   void