Update Request Cookies Type to Add undefined

[ryanagillie]

Updates the TypeScript types to add | undefined to a request's cookies.

This is a breaking change as previously cookies were typed to be a string, while now that cannot be assumed. (Possibly 7.1.0?)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[ryanagillie]

Are the tests okay? I didn't exactly know how to test the types other than throwing the expectType<string | undefined>(test); in the very first one. I know I had to non-null assert the request.cookies.test to make it work.

I did add runtime checks to show that cookies can be undefined.

[mcollina]

lgtm, this looks good!