Strip MF-Custom-Service header from request param, closes #475

Previously, the `MF-Custom-Service` header was including on the
`request` parameter passed to custom service functions. This usually
wasn't a problem, but if the request ever made it back to the
loopback server (e.g. via `caches.default.put(request, ...)`), it
would be forwarded back to the custom service instead of the
appropriate plugin router.

packages/miniflare/src/index.ts

@@ -421,6 +421,7 @@ export class Miniflare {
 		try {
 			const customService = request.headers.get(HEADER_CUSTOM_SERVICE);
 			if (customService !== null) {
+				request.headers.delete(HEADER_CUSTOM_SERVICE);
 				response = await this.#handleLoopbackCustomService(
 					request,
 					customService

packages/miniflare/test/index.spec.ts

@@ -60,6 +60,9 @@ test("Miniflare: web socket kitchen sink", async (t) => {
 			CUSTOM(request) {
 				// Testing dispatchFetch custom cf injection
 				t.deepEqual(request.cf, { country: "MF" });
+				// Testing `MF-Custom-Service` header removed:
+				// https://github.com/cloudflare/miniflare/issues/475
+				t.is(request.headers.get("MF-Custom-Service"), null);
 				// Testing WebSocket-upgrading fetch
 				return fetch(`http://localhost:${port}`, request);
 			},