code: public void renderTo(OutputStream os) throws BeetlException
  	{
  		ByteWriter_Byte byteWriter = new ByteWriter_Byte(os, cf.charset, ctx);
  		this.renderTo(byteWriter);
  	}
docs: 获取模板输出的文本,输出到OutputStream里,tempalte必须在二进制输出模式下才能使用
  
  @param os
  @throws BeetlException