title: BytesIOhas data with getvalue but not read
body: <p>I have the following</p>
  <pre><code>block_blob_service = BlobServiceClient(account, sas)
  container_client = block_blob_service.get_container_client(containerName)
  downloader = container_client.download_blob(filename)
  output_stream = io.BytesIO()
  downloader.download_to_stream(output_stream)
  contents = io.TextIOWrapper(output_stream, encoding='UTF-8')
  print(len(output_stream.read(100))) 
  print(len(contents.read(100))) 
  print(len(output_stream.getvalue())) 
  </code></pre>
  <p>This prints</p>
  <pre><code>[2021-03-12T05:24:46.043Z] 0
  [2021-03-12T05:24:46.045Z] 0
  [2021-03-12T05:24:46.048Z] 528672
  </code></pre>
  <p>I don't understand how this can happen – how am I supposed to use TextIOWrapper</p>