code: public Map<String, String> getCachedChallenge(HttpUrl url) {
          if (url == null) {
              return null;
          }
          String authority = getAuthority(url);
          authority = authority.toLowerCase(Locale.ENGLISH);
          return cachedChallenges.get(authority);
      }
docs: Uses authority to retrieve the cached values.
  
  @param url
  the url that is used as a cache key.
  @return cached value or null if value is not available.