Add opt-in 405 Method Not Allowed responses

[EduardoAC]

Summary

• Adds an opt-in methodNotAllowed Router option to return 405 Method Not Allowed when a route path matches but the HTTP method is unsupported.
• Sets an Allow header based on matched route methods (includes HEAD when GET is supported via existing helpers).
• Default behaviour remains unchanged (still falls through to 404 without the option).

Why

Addresses expressjs/express#2414 by providing a per-router configurable 405 response without breaking existing applications.

Tests

• npx mocha --reporter spec --check-leaks test/