Add urlEncode utility

[yandodov]

It can be handy to have a function like this:

function urlEncode(data: object) {
	let fields = [];

	for (let key in data) {
		let value = data[key];

		if (value && typeof value === "object") {
			value = JSON.stringify(value);
		}

		let encodedKey = encodeURIComponent(key);
		let encodedValue = encodeURIComponent(value);

		fields.push(encodedKey + "=" + encodedValue);
	}

	return fields.join("&");
}

...when dealing with application/x-www-form-urlencoded forms.