Fix multipart error on rails reload

While running GitLab in development mode, avatar uploads may fail after a Ruby file change and rails reload has occurred.

The error is:

A copy of Gitlab::Middleware::Multipart::Handler has been removed from the module tree but is still active!

Gitlab::Middleware::Multipart::Handler#open_file  lib/gitlab/middleware/multipart.rb, line 86
Gitlab::Middleware::Multipart::Handler#decorate_params_value  lib/gitlab/middleware/multipart.rb, line 73
block in Gitlab::Middleware::Multipart::Handler#with_open_files  lib/gitlab/middleware/multipart.rb, line 48
Gitlab::Middleware::Multipart::Handler#with_open_files  lib/gitlab/middleware/multipart.rb, line 39
Gitlab::Middleware::Multipart#call  lib/gitlab/middleware/multipart.rb, line 105
Gitlab::RequestProfiler::Middleware#call  lib/gitlab/request_profiler/middleware.rb, line 14

This fix, suggested by @stanhu appears to work well. Thanks for the suggestion @stanhu 👍

cc @annabeldunstone, who raised it in Slack