Simulate real behaviour of Stream in in-memory implementation of BlobClient

[michal-zatloukal]

The in-memory implementation of BlobClient does not require flushing the stream (presumably because it uses InMemoryStream?), which causes a discrepancy where tests pass but the production code fails without an explicit flush. Tests do not reveal this issue.

This code works with the in-memory implementation:

var output = await blobClient.OpenWriteAsync(true, cancellationToken: cancellationToken);
await input.CopyToAsync(output, cancellationToken);

but production code needs this:

var output = await blobClient.OpenWriteAsync(true, cancellationToken: cancellationToken);
await input.CopyToAsync(output, cancellationToken);
await output.FlushAsync();