code: public byte[] composeHeader(int commandId, int commandStatus,
              int sequenceNumber) {
          PDUByteBuffer buf = new PDUByteBuffer(commandId, commandStatus,
                  sequenceNumber);
          return buf.toBytes();
      }
docs: /*
  (non-Javadoc)
  
  @see org.jsmpp.util.PDUComposer#composeHeader(int, int, int)