fix(req.get): perform case-insensitive header lookup

[armanmikoyan]

Fixes #5288

This updates req.get() / req.header() to perform a case-insensitive lookup when a header is present under a non-lowercase key.

Previously, custom headers added directly to req.headers with mixed-case names, such as X-filipe, could not be read with req.get('x-filipe'). This change preserves the fast direct lookup path, then falls back to scanning header keys case-insensitively.

req.headers['X-filipe'] = 'filipe'

req.get('x-filipe') // before: undefined

After this change, the same lookup returns the header value:

req.headers['X-filipe'] = 'filipe'

req.get('x-filipe') // after: 'filipe'

A regression test was added for custom mixed-case request headers.

[krzysdz]

I won't close this yet, but in my opinion #5288 is the result of modifying Node.js API object in a way that violates requirements/assumptions stated in documentation and Express should not work around problems in code that uses it.

[bjohansebas]

I'm adding it as an idea for Express 6. It doesn't mean it'll be included—it's just something for us to discuss.