danbooru::http: factor out cache feature.

Fixes a bug with cookies stored by the `session` feature not being sent
with cached requests.

app/logical/danbooru/http/cache.rb

@@ -0,0 +1,30 @@
+module Danbooru
+  class Http
+    class Cache < HTTP::Feature
+      HTTP::Options.register_feature :cache, self
+
+      attr_reader :expires_in
+
+      def initialize(expires_in:)
+        @expires_in = expires_in
+      end
+
+      def perform(request, &block)
+        ::Cache.get(cache_key(request), expires_in) do
+          response = yield request
+
+          # XXX hack to remove connection state from response body so we can serialize it for caching.
+          response.flush
+          response.body.instance_variable_set(:@connection, nil)
+          response.body.instance_variable_set(:@stream, nil)
+
+          response
+        end
+      end
+
+      def cache_key(request)
+        "http:" + ::Cache.hash({ method: request.verb, url: request.uri.to_s, headers: request.headers.sort }.to_json)
+      end
+    end
+  end
+end