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.

test/test_helpers/reportbooru_helper.rb

@@ -1,6 +1,6 @@
 module ReportbooruHelper
   def mock_request(url, method: :get, status: 200, body: nil, http: Danbooru::Http.any_instance, **options)
-    response = HTTP::Response.new(status: status, body: body, version: "1.1")
+    response = HTTP::Response.new(status: status, body: body, version: "1.1", request: nil)
     http.stubs(method).with(url, **options).returns(response)
   end