Fix loader request on document POST requests (#9721)

* Fix loader request on document POST requests

* add changeset

* assert empty body

* Keep GET method for loader requests

* update changeset

.changeset/kind-dodos-shop.md

@@ -0,0 +1,5 @@
+---
+"@remix-run/router": patch
+---
+
+Persist `headers` on `loader` `request`'s after SSR document `action` request

packages/router/__tests__/router-test.ts

@@ -10990,7 +10990,13 @@ describe("a router", () => {
             ],
           },
         ]);
-        await query(createSubmitRequest("/child"));
+        await query(
+          createSubmitRequest("/child", {
+            headers: {
+              test: "value",
+            },
+          })
+        );
 
         // @ts-expect-error
         let actionRequest = actionStub.mock.calls[0][0]?.request;
@@ -11007,8 +11013,12 @@ describe("a router", () => {
         let childLoaderRequest = childLoaderStub.mock.calls[0][0]?.request;
         expect(rootLoaderRequest.method).toBe("GET");
         expect(rootLoaderRequest.url).toBe("http://localhost/child");
+        expect(rootLoaderRequest.headers.get("test")).toBe("value");
+        expect(await rootLoaderRequest.text()).toBe("");
         expect(childLoaderRequest.method).toBe("GET");
         expect(childLoaderRequest.url).toBe("http://localhost/child");
+        expect(childLoaderRequest.headers.get("test")).toBe("value");
+        // Can't re-read body here since it's the same request as the root
       });
 
       it("should support a requestContext passed to loaders and actions", async () => {

packages/router/router.ts

@@ -2257,7 +2257,11 @@ export function unstable_createStaticHandler(
     }
 
     // Create a GET request for the loaders
-    let loaderRequest = new Request(request.url, { signal: request.signal });
+    let loaderRequest = new Request(request.url, {
+      headers: request.headers,
+      redirect: request.redirect,
+      signal: request.signal,
+    });
     let context = await loadRouteData(loaderRequest, matches, requestContext);
 
     return {