code: public Site addHeader(String key, String value) {
          headers.put(key, value);
          return this;
      }
docs: Put an Http header for downloader. <br>
  Use {@link #addCookie(String, String)} for cookie and {@link #setUserAgent(String)} for user-agent. <br>
  
  @param key   key of http header, there are some keys constant in {@link HttpConstant.Header}
  @param value value of header
  @return this