code: function amf3encXmlDoc(str) {
      var sLen = amf3encUI29(str.length << 1);
      var buf = new Buffer(1);
      buf.writeUInt8(0x7, 0);
      return Buffer.concat([buf, sLen, new Buffer(str, 'utf8')]);
  }
docs: AMF3 Encode XMLDoc
  @param str
  @returns {Buffer}