Access log entries lack newline characters when using JS configuration.

[hongzhidao]

When using format in access_log with JS, each log doesn't end with \n.
For example:

{
        "listeners":
                {"*:80":
                        {"pass":"routes"}
                },
        "routes":[
                {"action":{"return":204}}
        ],
        "access_log": {
                "path": "/tmp/access.log",
                "format": "`test`"
        }
}

But it works well with non-JS format option.