title: ImageProperties.SaveImagePropertiesAsync() doesn't save changes
body: <p>I'm trying to save some properties into an image file in a Windows 10 UWP application, on the mobile phone.</p>
  
  <pre><code>var fileProperties = await file.Properties.GetImagePropertiesAsync();
  
  fileProperties.Rating = 25;
  fileProperties.Title = "Title";
  fileProperties.DateTaken = DateTime.Now;
  
  await file.Properties.SavePropertiesAsync();
  </code></pre>
  
  <p>For some reason, the properties are not saved.</p>
  
  <p>The file is created beforehand like this:</p>
  
  <pre><code>var file = await _sourceFolder.CreateFileAsync(pathToFile, CreationCollisionOption.ReplaceExisting);
  await bitmap.SaveToStorageFile(file);
  </code></pre>
  
  <p>where bitmap is of type WriteableBitmap.
  The image is saved to a file, but the properties are not.</p>
  
  <p>Does anyone know what I'm not doing correctly? There is no exception, no message about why it doesn't succceed.</p>