Fix client-side errors caused by cached prefetch requests #442
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https://github.com/vercel/next.js/blob/canary/packages/next/server/next-server.ts#L1996
This code shows that when the client sends a request with an
x-middleware-prefetchheader, the server will return an empty JSON object with anx-middleware-skipheader. If we cache these requests, will cause client error when doing client side render. We need to skip these requests when caching.