I'm trying to understand some code here:
I'm trying to understand how it invalidates the cache:
// Stop using the cache if there is a mutating call.
    if (req.method !== 'GET' && req.method !== 'HEAD') {
      this.isCacheActive = false;
      this.invalidateCacheEntry(req.url);
    }
How do you get a situation when req.method !== 'GET'? I only use GET requests.