code: function nonce(len) {
    var buf = []
      , chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789'
      , charlen = chars.length;
  
    for (var i = 0; i < len; ++i) {
      buf.push(chars[Math.random() * charlen | 0]);
    }
  
    return buf.join('');
  }
docs: Return a unique nonce with the given `len`.
  
  utils.uid(10);
  // => "FDaS435D2z"
  
  CREDIT: Connect -- utils.uid
  https://github.com/senchalabs/connect/blob/1.7.1/lib/utils.js
  
  @param {Number} len
  @return {String}
  @api private