[feat] HTTP singleton header validations

## Describe the feature

follow-up to https://github.com/koajs/koa/pull/1899

if an HTTP header is a singleton, we should have stricter validations and disallow setting multiple fields.

Start by reading the follow-up pull request at https://github.com/koajs/koa/pull/1899 and tracing the HTTP header handling it changed. Identify how singleton headers are currently accepted, then define validation behavior that rejects multiple fields; done means singleton headers cannot be set more than once and the relevant behavior is covered by tests.

Written by the indexing model from the issue text.