fix: keep the Content-Disposition filename ASCII for non-ASCII file names

[RaphaelFakhri]

Fixes #205. Files with non-ASCII names such as brasão.txt or DALL·E.png fail with ERR_INVALID_CHAR and the request never gets a valid response.

Problem

content-disposition keeps ISO-8859-1 characters in the plain filename parameter. Node.js rejects the header when the response also sets Content-Length, so writeHead() throws ERR_INVALID_CHAR for the Content-Disposition header.

Changes

• Passes an ASCII-only fallback to contentDisposition(). Non-ASCII characters in the plain filename parameter become ?, and the full name is sent in filename* (RFC 5987). ASCII names produce the same header as before.
• Adds an integration test that serves files named with Latin-1, a middle dot, and CJK characters.

Testing

yarn run build-views && npx jest --forceExit test/integration.test.js

Before the change, the new test fails with status 500 and the other 70 tests pass. After the change, all 71 tests pass and zeit-eslint is clean.