Configured "HttpHeaders" are not sent by Docker daemon

Issue:

Configured "HttpHeaders" are not sent to the repository.

Documentation:

engine/reference/commandline/cli.md

Docker version

Docker version 19.03.13, build 4484c46d9d

Platform:

Ubuntu 18 & MacOS

Reproducing:

I am running an HTTPS proxy, which performs the SSL termination.
It receives the requests from the daemon and forwards an HTTPS request to the GCR project. The proxy logs all HTTP headers it receives.

1. Added
   "HttpHeaders": { "MyHeader": "MyValue" } to ~/.docker/config.json, as documented

2. Execute
   docker --config ~/.docker pull https://my_proxy/my_image:tag

3. Proxy logs all received log HTTP Headers. "MyHeader" is not found

INFO[8654] Client header "Connection": "close"
INFO[8654] Client header "User-Agent": "docker/19.03.12 go/go1.13.10 git-commit/48a66213fe kernel/4.19.76-linuxkit os/linux arch/amd64 UpstreamClient(Docker-Client/19.03.12 \\(darwin\\))"
INFO[8654] Client header "Accept-Encoding": "gzip"```

Start with engine/reference/commandline/cli.md and review the documented HttpHeaders setting in ~/.docker/config.json. Reproduce the docker pull through the HTTPS proxy, then determine whether the documentation or Docker daemon behavior is incorrect; done means the issue has a confirmed resolution and the relevant documentation or component reference is updated.