code: public static long copy(Reader reader, Writer writer, int bufferSize) throws IORuntimeException {
  		return copy(reader, writer, bufferSize, null);
  	}
docs: 将Reader中的内容复制到Writer中
  
  @param reader Reader
  @param writer Writer
  @param bufferSize 缓存大小
  @return 传输的byte数
  @throws IORuntimeException IO异常