Allow access request headers both in Express.js and Web Api (Next.js) manner

[eugef]

Fixes #316

[Copilot]

Pull Request Overview

This PR enables dual support for accessing request headers in both Express.js and Web API (Next.js) styles by introducing a new Headers implementation and updating the mockRequest object accordingly.

• Fixed a spelling mistake in test descriptions.
• Updated tests to verify header access via the new Headers interface.
• Integrated the new createHeaders function into mockRequest and provided comprehensive tests in headers.spec.js.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

File	Description
test/lib/mockRequest.spec.ts	Spelling fix in test description and updated assertions for headers.get.
test/lib/headers.spec.js	Added tests for the new Headers implementation and its methods.
lib/mockRequest.js	Updated to use createHeaders for header initialization.
lib/headers.js	New file implementing a Headers proxy with both Express.js and Web API support.

Comments suppressed due to low confidence (1)

test/lib/mockRequest.spec.ts:45

• Correct the typo in the test description: change 'shoud' to 'should'.

it('shoud initialize with default options', () => {