Bug: `payload` function in `composite.go` should set `httpHeaders` field to `map[string]string` instead of `map[string][]string`

[sfc-gh-ntrinh]

I was using this for a project and noticed that Salesforce expects a map[string]string instead of map[string][]string for the httpHeaders field in composite subrequests.

https://developer.salesforce.com/docs/atlas.en-us.api_rest.meta/api_rest/dome_composite_record_manipulation.htm

When I use

            "httpHeaders": {
                "Some-Header": ["defg"]
            }

I get the error

[
    {
        "message": "Invalid type provided for value in 'httpHeaders'",
        "errorCode": "JSON_PARSER_ERROR"
    }
]

and the request works when I use

            "httpHeaders": {
                "Some-Header": "defg"
            }