docs: public Response changeHeaders(String bucket, String key, Map<String, String> headers)
              throws QiniuException {
          String resource = encodedEntry(bucket, key);
          String path = String.format("/chgm/%s", resource);
          for (String k : headers.keySet()) {
              String encodedMetaValue = UrlSafeBase64.encodeToString(headers.get(k));
              path = String.format("%s/x-qn-meta-!%s/%s", path, k, encodedMetaValue);
          }
          return rsPost(bucket, path, null);
      }