danbooru::http: support tracking cookies between requests.

Allow cookies to be saved and sent back when making several requests in
a row. Usage:

    http = Danbooru::Http.use(:session)

    # saves the foo=42 cookie sent by the response.
    http.get("https://httpbin.org/cookies/set/foo/42")

    # sends back the foo=42 cookie from the previous request.
    http.get("https://httpbin.org/cookies")

app/logical/danbooru/http/session.rb

@@ -0,0 +1,37 @@
+module Danbooru
+  class Http
+    class Session < HTTP::Feature
+      HTTP::Options.register_feature :session, self
+
+      attr_reader :cookie_jar
+
+      def initialize(cookie_jar: HTTP::CookieJar.new)
+        @cookie_jar = cookie_jar
+      end
+
+      def perform(request)
+        add_cookies(request)
+        response = yield request
+        save_cookies(response)
+        response
+      end
+
+      def add_cookies(request)
+        cookies = cookies_for_request(request)
+        request.headers["Cookie"] = cookies if cookies.present?
+      end
+
+      def cookies_for_request(request)
+        saved_cookies = cookie_jar.each(request.uri).map { |c| [c.name, c.value] }.to_h
+        request_cookies = HTTP::Cookie.cookie_value_to_hash(request.headers["Cookie"].to_s)
+        saved_cookies.merge(request_cookies).map { |name, value| "#{name}=#{value}" }.join("; ")
+      end
+
+      def save_cookies(response)
+        response.cookies.each do |cookie|
+          cookie_jar.add(cookie)
+        end
+      end
+    end
+  end
+end