containers list api route response header is text/plain instead of application/json

[iongion]

/kind bug

Description

Response content-type header is not application/json for containers list endpoint, although negotiation requests it.
Some client libraries do attempt deserializing automatically to their data structures. Although it can be mitigated as an exception for this route, it should respect the negotiation. (v3 and v4 have the same behavior)

Steps to reproduce the issue:

1. base url v3.0.0 - curl -v -X GET --unix-socket "/run/user/1000/podman/podman.sock" "http://d/v3.0.0/libpod/containers/json" -H "Accept: application/json" -H "Content-Type: application/json"

2. base url v4.0.0 - curl -v -X GET --unix-socket "/run/user/1000/podman/podman.sock" "http://d/v4.0.0/libpod/containers/json" -H "Accept: application/json" -H "Content-Type: application/json"

Describe the results you received:

> GET /v4.0.0/libpod/containers/json HTTP/1.1
...
< Content-Type: text/plain; charset=us-ascii
< Libpod-Api-Version: 4.1.0
...

Describe the results you expected:

> GET /v4.0.0/libpod/containers/json HTTP/1.1
...
< Content-Type: application/json; charset=us-ascii
< Libpod-Api-Version: 4.1.0
...

If any use, issued the same for docker api and it does have application/json as response content-type header

docker --version
Docker version 20.10.16, build aa7e414fdc

curl -v -X GET --unix-socket "/var/run/docker.sock" "http://localhost/containers/json" -H "Accept: application/json" -H "Content-Type: application/json"
Note: Unnecessary use of -X or --request, GET is already inferred.
*   Trying /var/run/docker.sock:0...
* Connected to localhost (/run/docker.sock) port 80 (#0)
> GET /containers/json HTTP/1.1
> Host: localhost
> User-Agent: curl/7.83.1
> Accept: application/json
> Content-Type: application/json
> 
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 OK
< Api-Version: 1.41
< Content-Type: application/json
< Docker-Experimental: false
< Ostype: linux
< Server: Docker/20.10.16 (linux)
< Date: Fri, 17 Jun 2022 15:57:22 GMT
< Transfer-Encoding: chunked

IMPORTANT EDIT - It only happens when the list of containers is empty, as soon as at least one is present it respects!

[Luap99]

#14662 should fix it