function base64(url, obj, _, fn){
  if (3 == arguments.length) fn = _;
  var prefix = exports.prefix;
  obj = encode(JSON.stringify(obj));
  obj = encodeURIComponent(obj);
  url += '?' + prefix + '=' + obj;
  jsonp(url, { param: exports.callback }, function(err, obj){
    if (err) return fn(err);
    fn(null, {
      url: url,
      body: obj
    });
  });
}
Send the given `obj` to `url` with `fn(err, req)`.

@param {String} url
@param {Object} obj
@param {Function} fn
@api private