Recommended fix for CORs immutable headers issue

Fixes kwhitley#261 by creating a new response with mutable headers and avoiding duplicating the body. The correct fix for what kwhitley@74f63ab#diff-a1624121bc1fee18f8a5c2b2f907f01ba9c56a50068007e4c5da2911e91ea6df was trying to achieve.

src/cors.ts

@@ -78,7 +78,7 @@ export const cors = (options: CorsOptions = {}) => {
       || response.status == 101
     ) return response
 
-    return appendHeadersAndReturn(response.clone(), {
+    return appendHeadersAndReturn(new Response(response.body, response), {
       'access-control-allow-origin': getAccessControlOrigin(request),
       'access-control-allow-credentials': credentials,
     })