Implement Headers/Request

[mattvb91]

Finish implementing the Request class based on the API:

https://caddyserver.com/docs/modules/http.handlers.headers

The current implementation only covers delete: Request

The add & set are going to be pretty much the exact same setup as the existing delete.

The replace part will be similar but will need its own class with associated toArray method.

Bonus points for integration tests posting against the current caddy instance running in the tests.