danbooru::http: support automatically following redirects.

Replace http.rb's builtin redirect following option with our own
redirect follower. This fixes an issue with http.rb losing cookies after
following a redirect.

app/logical/danbooru/http/redirector.rb

@@ -0,0 +1,40 @@
+# A HTTP::Feature that automatically follows HTTP redirects.
+# https://developer.mozilla.org/en-US/docs/Web/HTTP/Redirections
+
+module Danbooru
+  class Http
+    class Redirector < HTTP::Feature
+      HTTP::Options.register_feature :redirector, self
+
+      attr_reader :max_redirects
+
+      def initialize(max_redirects: 5)
+        @max_redirects = max_redirects
+      end
+
+      def perform(request, &block)
+        response = yield request
+
+        redirects = max_redirects
+        while response.status.redirect?
+          raise HTTP::Redirector::TooManyRedirectsError if redirects <= 0
+
+          response = yield build_redirect(request, response)
+          redirects -= 1
+        end
+
+        response
+      end
+
+      def build_redirect(request, response)
+        location = response.headers["Location"].to_s
+        uri = HTTP::URI.parse(location)
+
+        verb = request.verb
+        verb = :get if response.status == 303 && !request.verb.in?([:get, :head])
+
+        request.redirect(uri, verb)
+      end
+    end
+  end
+end