Refactor `#cookie` API

[ixti]

1. HTTP::Response#cookies must return Array<HTTP::Cookies> - not jar.
2. HTTP::Client#cookies should be either eliminated (IMO that's the best options) or become just a simple way of adding Cookie: header with no magic of cookies concatenation involved.

/cc @httprb/core

[ixti]

Right now #cookies passing is somewhat weird (IMO):

def cookie_header(response)
  response.parse(:json).dig("headers", "Cookie")
end

http = HTTP.cookies(:a => 1, :b => 2).cookies(:a => 3)
cookie_header http.get("https://httpbin.org/headers")
# => "a=3; b=2"

cookie_header http.get("https://httpbin.org/headers", :cookies => { :a => 4 })
# => "4"

http = http.headers("Cookie" => "a=1;f=6")
cookie_header http.get("https://httpbin.org/headers")
# "a=1;f=6; a=3; b=2"

cookie_header http.get("https://httpbin.org/headers", :cookies => { :a => 4 })
# "a=1;f=6; 4"

[tarcieri]

The CookieJar integration is definitely screwed up, and belongs on a hypothetical session type (#306), IMO.

I would agree with your assessment that this API needs changed and is presently awkward/asymmetrical.

[ixti]

Yes. Full cookie management belongs to session class. Here we should make it straight forward and simple to just pass cookies in and out.