Fix multipart boundaries containing equals signs

[raashish1601]

Fixes #1051.

Multer used type-is to detect multipart requests before handing the request to Busboy. type-is rejects unquoted multipart boundary parameters containing =, even though those boundaries can be parsed by the multipart stream itself. The result was that Multer skipped the request entirely.

This changes the early multipart check to avoid parsing parameters and only inspect the media type while still honoring type-is.hasBody(req). Before constructing Busboy, it normalizes only unquoted boundary values containing = by quoting the boundary in the headers passed to Busboy. The original request headers are not mutated.

Validation:

• npx mocha --reporter spec --exit --check-leaks test/fields.js
• npm run lint
• npm test was also run; it still fails on existing Windows fixture-size/path assertions unrelated to this change, while the new fields regression passed.