Fix direct access to the headers

[eugef]

Fixes #318

[Copilot]

Pull Request Overview

This PR addresses issue #318 by fixing direct header access, ensuring that headers are accessed consistently using both the Web API and Express-style interfaces.

• Updates test cases in both mockRequest and headers specs to validate direct header access via various methods.
• Refactors header assignments and retrievals in lib/mockRequest.js and lib/headers.js for case‐insensitive access.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
test/lib/mockRequest.spec.ts	Added tests for direct header access using different methods.
test/lib/headers.spec.js	Updated test expectations to use standardized header casing.
lib/mockRequest.js	Removed lower-casing conversion during header creation to rely on proxy behavior.
lib/headers.js	Adjusted the proxy to enforce case-insensitive header access consistently.