rest client is missing headers

If I try to make the request GET /_cluster/health through the REST interface, I get:

{
  "error": "Content-Type header is missing",
  "status": 406
}

I'm running AWS elasticsearch 6.7, and using a reverse proxy to get the signed request that AWS needs. If I curl to the proxy I get the expected response so I'm assuming the issue is in cerebro's rest client, and not the proxy (the curl command was copied from cerebro's rest interface).

Odly enough, the proxy is actually receiving this very same requests from cerebro's normal operation and it works well (for example /_cluster/health).

This is the proxy: https://github.com/mozilla-services/aws-signing-proxy