fix js fetch headers (##589)

test/js.js

@@ -0,0 +1,20 @@
+async function onRequest() {
+
+
+    const fetchResponse = await fetch('https://httpbin.org/anything');
+    console.log(fetchResponse.headers);
+    console.log(await  fetchResponse.text());
+    console.log(  fetchResponse.body);
+
+    const response = {
+        statusCode: 200,
+        body: fetchResponse.body,
+        headers: fetchResponse.headers
+    };
+    return response;
+}
+
+onRequest().then( response => {
+    console.log('Response:', response);
+
+})