Describe the bug. A clear and concise description of what the bug is.

After upgrading to v4.0.2 from v4.0.0, Centrifugo stopped passing the configured proxy headers to the proxy endpoint.
I noticed #558 in v4.0.2 changelogs and changed the configured headers in config.json to have all of their letters lowercased. (E.g. from Foo-Bar-Baz to foo-bar-baz.)

After the change, Centrifugo started passing the proxy headers again. (Double-checked this by reverting and re-applying the config change to make sure the problem is with the uppercase letters.)

Versions

Centrifugo version is 4.0.2
Client library used is centrifuge-go of version v0.9.3
Operating system is macOS

Steps to Reproduce How can the bug be triggered?

1. Add a connect proxy configuration similar to this:

{
  "connect_proxy_name": "connect"
  "proxies": [
    {
      "name": "connect",
      "endpoint": "https://cent-proxy.example.com/connect",
      "http_headers": ["Foo-Bar-Baz", "Qux-Fred-Thud"]
    }
 }

2. Send a connect request to the Centrifugo server with Foo-Bar-Baz and Qux-Fred-Thud headers set.

Expected behavior What output or behaviour were you expecting instead?

Centrifugo should pass Foo-Bar-Baz and Qux-Fred-Thud headers to the proxy endpoint, but it doesn't.