fix: respect res header immutability (#10554)

Properly respect and merge res headers.

packages/payload/src/utilities/handleEndpoints.ts

@@ -8,6 +8,7 @@ import type { PayloadRequest } from '../types/index.js'
 
 import { createPayloadRequest } from './createPayloadRequest.js'
 import { headersWithCors } from './headersWithCors.js'
+import { mergeHeaders } from './mergeHeaders.js'
 import { routeError } from './routeError.js'
 
 const notFoundResponse = (req: PayloadRequest) => {
@@ -215,14 +216,11 @@ export const handleEndpoints = async ({
     }
 
     const response = await handler(req)
-
-    if (req.responseHeaders) {
-      for (const [key, value] of req.responseHeaders) {
-        response.headers.append(key, value)
-      }
-    }
-
-    return response
+    return new Response(response.body, {
+      headers: mergeHeaders(req.responseHeaders ?? new Headers(), response.headers),
+      status: response.status,
+      statusText: response.statusText,
+    })
   } catch (err) {
     return routeError({
       collection,