refactor: based on CR advice

Author: chenxi-null

src/client/sse.test.ts

@@ -717,7 +717,7 @@ describe("SSEClientTransport", () => {
         eventSourceInit: {
           fetch: (url, init) => {
             return fetch(url, { ...init, headers: {
-              ...init?.headers,
+              ...(init?.headers instanceof Headers ? Object.fromEntries(init.headers.entries()) : init?.headers),
               'X-Custom-Header': 'custom-value'
             } });
           }

src/client/sse.ts

@@ -130,9 +130,7 @@ export class SSEClientTransport implements Transport {
             headers.set("Accept", "text/event-stream");
             const response = await fetchImpl(url, {
               ...init,
-              headers: {
-                ...Object.fromEntries(headers.entries()),
-              }
+              headers,
             })
 
             if (response.status === 401 && response.headers.has('www-authenticate')) {