[http/cookie] Add support for adding "Cookie" header

**Is your feature request related to a problem? Please describe.**

I am trying to send a fetch request to a server from Deno and memorize cookies set by the server in the response.

The standard library is missing a function to assign a `Cookie` header to the `Headers` object. There are functions to read `Cookie` headers and read and assign `Set-Cookie` headers, but there is currently no way to set `Cookie` headers.

**Describe the solution you'd like**

A `setCookie(headers: Headers, cookie: Cookie)` method could set a `Cookie` header. There is one issue though, a method with this name already exists and it sets the `Set-Cookie` header.

There is a naming inconsistency in the cookie standard library:
- `getCookies()` - returns all `Cookie` headers
- `getSetCookies()` - returns all `Set-Cookie` headers
- `setCookie(...)` - currently sets a `Cookie` header, but should set the `Set-Cookie` header should the naming of functions be consistent.

**Describe alternatives you've considered**

I have tried to set the `Cookie` header directly, but it is not convenient at the moment, as there is no way to easily reuse the `Cookie` parsed object from the `getSetCookies()` method and use that as a value to set cookie in headers directly. It would be possible with the `toString` method, which is private to the `cookies.ts` module.

I'll be happy to send a PR once it is decided whether to implement this and how to solve the naming issue.