I have the same issue with Rails 5.1

There was a fix for this at the end of April, but a new release wasn't made. Are you two downloading the gem straight from this repository?

If the git version fixes this issue, I'll go ahead and release an update.

I think it's still not fixed. I thought it was, because the header is correct when I request a file with curl -I --compressed. However, loading that same file in Chrome gets me the incorrect header that people have reported.

I just tried the fork from PR #40 by @bookwitty and it has problems as well. The header is set as a hash. In other words, this line:
{ 'Cache-Control' => app.config.public_file_server.headers }

results in this header:
Cache-Control:{"Cache-Control"=>"public, max-age=31536000"}

15 days ago I tried from the repo and the released version but had no success. Cache control was always stuck to 24 hours.

@benoitongit Try adding this in config/initializers/heroku_deflater_patch.rb:

module HerokuDeflater
  class CacheControlManager
    def cache_control_headers
      if rails_version_5?
        app.config.public_file_server.headers
      else
        app.config.static_cache_control
      end
    end
  end
end

@romanbsd can you please update it on Rubygems?

Seems like this still never actually made it into a release?

Rubygems version hasn't been updated since January 2017 (version 0.6.3).

Use the following to use the github version:
gem 'heroku-deflater', git: 'https://github.com/romanbsd/heroku-deflater'

The issue is that commit f4d5b4c while fixing one thing, broke another, and also the specs are failing. I hadn't had time to look into it, and then forgot about it completely. If someone is willing to check, I'll be more than happy to receive a PR and release a new version.