title: Mock of IBinder fails when unit testing Activity Function
body: <p>I am trying to test an activity function which has the following definition:</p>
  
  <pre><code>[FunctionName(nameof(LoadReferenceFromBlobStorage))]
  public static async Task&lt;string&gt; Run([ActivityTrigger] string blobName,
      IBinder binder,
      ILogger log)
  {
      StorageAccountAttribute storageAccountAtt = new StorageAccountAttribute("AzureWebJobsStorage");
      CloudStorageAccount storageAccount = await binder.BindAsync&lt;CloudStorageAccount&gt;(storageAccountAtt, CancellationToken.None);
      CloudBlobClient client = storageAccount.CreateCloudBlobClient();
  
      //...
  }
  </code></pre>
  
  <p>I mock the <code>IBinder</code> in the unit test as:</p>
  
  <pre><code>[TestMethod]
  public async Task GetReference()
  {
      var attribute = new StorageAccountAttribute("UseDevelopmentStorage=true;");
      var mock = new Mock&lt;IBinder&gt;();
      CloudStorageAccount mockedResult = null;
      CloudStorageAccount.TryParse("UseDevelopmentStorage=true;", out mockedResult);
      mock.Setup(x =&gt; x.BindAsync&lt;CloudStorageAccount&gt;(attribute, CancellationToken.None))
              .ReturnsAsync(mockedResult);
  
      ILogger logger = NullLoggerFactory.Instance.CreateLogger("log");
      var res = await LoadReferenceFromBlobStorage.Run("name", mock.Object, logger);
  
      //...
  }
  </code></pre>
  
  <p>The test calls the activity successfully but the result of <code>binder.BindAsync</code> is always <em>null</em>.</p>
  
  <p>Am I missing something?</p>