DownloadUrl should store bytes, not BSON

At some point DownloadUrl decided to use the Object methods which is hella wrong

Test Code (this is a total PITA to test in the normal test runner because WPF will cry if we try to spin it up):

```cs
        public MainWindow()
        {
            InitializeComponent();

            BlobCache.ApplicationName = "Test";
            this.Loaded += async (o, e) => {
                await BlobCache.LocalMachine.InvalidateAll();

                var image = await BlobCache.LocalMachine.LoadImageFromUrl("http://i.creativecommons.org/l/by-nc-sa/3.0/us/88x31.png");
                this.image.Source = image.ToNative();

                await Task.Delay(2500);

                // From cache
                image = await BlobCache.LocalMachine.LoadImageFromUrl("http://i.creativecommons.org/l/by-nc-sa/3.0/us/88x31.png");
                this.image.Source = image.ToNative();

                await Task.Delay(2500);

                // Force fetch
                image = await BlobCache.LocalMachine.LoadImageFromUrl("http://i.creativecommons.org/l/by-nc-sa/3.0/us/88x31.png", true);
                this.image.Source = image.ToNative();

                await Task.Delay(2500);

                // Load Image
                image = await BlobCache.LocalMachine.LoadImage("http://i.creativecommons.org/l/by-nc-sa/3.0/us/88x31.png");
                this.image.Source = image.ToNative();

                await Task.Delay(2500);

                MessageBox.Show("Done!");
            };
        }
```

Fixes #196