How to send a json representation of the data but with custom Accept header?

I am doing a post to Github API which requires custom Accept header and json representation.
This is what I have

    var options = {
        headers: {
            Authorization: 'token <token>',
            Accept: 'application/vnd.github.everest-preview+json'
        },
    }
    var data = {
        event_type: "autotest",
        client_payload: {"componentToRun": "app"}
    }
    try {
        const res = await needle("post", 'https://api.github.com/repos/myrepo/reponame/dispatches',
            data,  options);
    } catch(e) {
    }

If I set json: true in options, the accept header is changed to Application/Json which is not what I want. I simply want to send the data as json without it touching my headers.

Start by tracing how the post request handles the json option and merges or replaces the supplied Accept header. Reproduce the GitHub API example from the issue, then inspect the relevant request-option tests if available. Done means JSON is sent while the custom Accept header remains unchanged.