Added "no-transform" to Cache-Control header

Some proxies buffers SSE events to compress the stream, delaying the browser event reception. The `no-transform` `Cache-Control` directive forbids this bad behavior. More info at:

facebook/create-react-app#1633
vercel/next.js#9965 (comment)

(cherry picked from commit 69fb11f)

src/plugin.ts

@@ -12,7 +12,7 @@ export const plugin: Plugin<Server, IncomingMessage, ServerResponse, SsePluginOp
           this.res.write(serializeSSEEvent({retry: options.retryDelay || 3000}));
           this.type("text/event-stream")
             .header("Connection", "keep-alive")
-            .header("Cache-Control", "no-cache");
+            .header("Cache-Control", "no-cache,no-transform");
           toStream(transformAsyncIterable(source)).pipe(this.res);
         });
     };