Middleware should not add trailing slashes to non-data requests in static dir

[serhalp]

In sites with trailing slashes enabled, requests should not add a trailing slash when they're for non-data static files.

Data

The following is parsed automatically by the Next.js repo e2e test report generator.

test case: https://github.com/vercel/next.js/blob/canary/test/e2e/middleware-trailing-slash/test/index.test.ts#L436-L437
test: test/e2e/middleware-trailing-slash/test/index.test.ts
reason: Middleware should not add trailing slashes to non-data requests in static dir