test.js

var fs = require("fs");
var https = require("https");
var bearer = "pXZ8xSWvjZjE4UaNntVsCwWPg:ziSswccgj8xm7TGrJ0GcEoVl2tgPaMiNYOWzQeDIhpFTJIMSro"
var base64ed = new Buffer(bearer).toString("base64");

var options = {
	port: 443,
	hostname: "api.twitter.com",
	//	path: "/1.1/search/tweets.json?" + encodeURIComponent("q='social media'"),
	path: "/oauth2/token",
	method: "post",
	key: fs.readFileSync("./testssl.key"),
	cert: fs.readFileSync("./testcert.cert"),
	//	headers: {
	//		"content-type": "application/x-www-form-urlencoded;charset=UTF-8"
	//
	//	},
	auth: base64ed + '"content-type": "application/x-www-form-urlencoded;charset=UTF-8"'
}

var req = https.request(options, res => {
	console.log(res.req._header);
	res.on("data", d => {
		console.log(d.toString());
	})
})
req.on("error", e => {
	console.log(e);
});
req.write("grant_type=client_credentials");
req.end();