code: function b64utob64(s) {
      if (s.length % 4 == 2) s = s + "==";
      else if (s.length % 4 == 3) s = s + "=";
      s = s.replace(/-/g, "+");
      s = s.replace(/_/g, "/");
      return s;
  }
docs: convert a Base64URL encoded string to a Base64 encoded string.<br/>
  Example: "ab-c3f_" &rarr; "ab+c3f/=="
  @param {String} s Base64URL encoded string
  @return {String} Base64 encoded string