HaveHttpStatus uses Rack's public API

Instead of accessing a Constant that isn't a documented part of
the Rack API, get the status code for a status name using Rack's
supported public API.

lib/rspec/rails/matchers/have_http_status.rb

@@ -216,11 +216,7 @@ def pp_status(status, code)
           # @see Rack::Utils::SYMBOL_TO_STATUS_CODE
           # @raise [ArgumentError] if an associated code could not be found
           def set_expected_code!
-            @expected ||=
-              Rack::Utils::SYMBOL_TO_STATUS_CODE.fetch(expected_status) do
-                raise ArgumentError,
-                      "Invalid HTTP status: #{expected_status.inspect}"
-              end
+            @expected ||= Rack::Utils.status_code(expected_status)
           end
         end