title: Check if URL exist in service worker cache
body: <p>I have written below code for check if particular URL is already in service worker cache or not ? But Even if the URL is not present in cache, it returns/consoles "Found in cache". </p>
  
  <pre><code>var isExistInCache = function(request){
      return caches.open(this.cacheName).then(function(cache) {
          return cache.match(request).then(function(response){
              debug_("Found in cache "+response,debug);
              return true;
          },function(err){
              debug_("Not found in cache "+response,debug);
              return false;
          });
        })
  }
  </code></pre>
  
  <p>Calling above function as </p>
  
  <pre><code>cache.isExistInCache('http://localhost:8080/myroom.css').then(function(isExist){
          console.log(isExist);
      })
  </code></pre>