gems: upgrade http-cookie to fix Rails 6.1 bug.

Upgrade the http-cookie gem to a personal fork containing a bugfix for a
http-cookie bug that is triggered by Rails 6.1.

The bug is that HTTP::Cookie objects raise an exception if they're
compared against non-cookie objects. This bug gets triggered when
the Nijie source strategy calls `Rails.cache.fetch` to cache the
Nijie login cookie. `Rails.cache.fetch` ends up calling
ActiveSupport::Cache::Store::Entry#dup_value!, which compares the cookie
with `true`, which triggers the exception.

The http-cookie gem hasn't been updated for 4 years, so we're stuck
patching the library ourselves.

Gemfile

@@ -41,6 +41,7 @@ gem 'scenic'
 gem 'ipaddress_2'
 gem 'http'
 gem 'activerecord-hierarchical_query'
+gem 'http-cookie', git: "https://github.com/danbooru/http-cookie"
 gem 'pundit'
 gem 'mail'
 gem 'nokogiri'