Upgrade http.rb gem to 5.0.4.

Fixes a bug where the Foundation source strategy failed because http.rb
automatically sent a `Content-Length: 0` header with all GET requests,
which caused Foundation to return a 400 Bad Request error. This behavior
was fixed in http.rb 5.x.

http.rb 5.x has a breaking change where it now includes the request object
inside the response object, which we have to handle in a few places.

app/logical/danbooru/http/cache.rb

@@ -19,6 +19,9 @@ module Danbooru
           response.flush
           response.body.instance_variable_set(:@connection, nil)
           response.body.instance_variable_set(:@stream, nil)
+          response.instance_exec { @request = request.dup }
+          response.request.instance_exec { @uri_normalizer = nil }
+          response.request.body.instance_exec { @source = nil }
 
           response
         end