Fix prefetched data being downloaded twice

[brookslybrand]

Hovering a menu link prefetches the next page's data, but clicking it downloads the data again. This happens on production today, not just in the RSC PRs.

The doc route sets Vary: Accept so the same URL can serve HTML or Markdown (#248). Route headers also apply to .data responses, so those get Vary: Accept too. The prefetch is a <link rel="prefetch">, which Chrome sends with Accept: text/html,…, while the navigation is a fetch() with Accept: */*. Because of Vary: Accept, the browser won't reuse the prefetched response for a request with a different Accept header, so it downloads the data again.

Data URLs (.data, and .rsc once #259 lands) are never content-negotiated, so the server now ignores Accept on GET/HEAD requests to them and drops Accept from Vary on the response. It's a fetch-router middleware in server.ts, not React Router middleware, because React Router strips the extension before its middleware and headers run, so they can't tell a data request from a document request. The app request is now built from context.headers, so header changes made by middleware reach React Router.

Removing Accept from the request matters, not only the Vary change. Right now curl -H "Accept: text/markdown" https://reactrouter.com/start/framework/routing.data returns Markdown. With Vary: Accept gone, Fly's Varnish could cache that Markdown under the .data URL and serve it to everyone's navigations.

Tested on a production build:

Request	Before	After
/start/framework/routing.data	Vary: Cookie, Accept	Vary: Cookie
.data with Accept: text/markdown	Markdown	Data
/start/framework/routing	Vary: Cookie, Accept	Unchanged
/start/framework/routing with Accept: text/markdown	Markdown	Unchanged
Prefetch, then fetch() of the same .data URL	2 server hits	1 server hit (fetch served from cache)