code: protected void generateHeader() {
          conn.setRequestProperty("Accept-Charset", charset);
          // text/json
          // application/x-www-form-urlencoded
          // text/plain
          conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
          conn.setRequestProperty("Connection", "Keep-Alive");
          if (header != null)
              for (String key : header.getHeaders().keySet()) {
                  conn.setRequestProperty(key, header.getHeaders().get(key));
              }
      }
docs: 构建请求头