Fix playback service doesn't work with azure CDN with media optimization

feature enabled.

For azure CDN with media optimization enabled, if the media file is
missing, the CDN will request a very big range. In the original
implementation, playback service doesn't honor the real content length
instead, it will return the request range. This incorrect behavior will
make the cdn return network protocol error.

tools/PlaybackService/src/Controllers/PlaybackController.cs

@@ -425,21 +425,20 @@ private async Task GetOtherFilesAsync(BlobClient blob, long contentLength)
             throw new Exception($"Can't download the blob {blob.Uri}");
         }
 
-        // Write the response.
-        if (range != null)
-        {
-            var readedLength = range.Value.Length ?? contentLength - range.Value.Offset;
-            Response.ContentLength = readedLength;
+        // Copy the http status code and response headers.
+        var blobServerResponse = blobStreamResponse.GetRawResponse();
+        Response.StatusCode = blobServerResponse.Status;
 
-            var contentRange = $"{range.Value.Offset}-{readedLength + range.Value.Offset - 1}/{contentLength}";
-            logger.LogDebug($"Add header Content-Range: {contentRange}");
-            Response.Headers.Add("Content-Range", contentRange);
-        }
-        else
+        // For security, we only expose the following headers:
+        foreach (var allowHeader in new[] { "Content-Length", "Content-Type", "Date", "Accept-Ranges", "Content-MD5", "ETag", "Last-Modified" })
         {
-            Response.ContentLength = contentLength;
+            if (blobServerResponse.Headers.TryGetValue(allowHeader, out var header))
+            {
+                Response.Headers.TryAdd(allowHeader, header);
+            }
         }
 
+        // Copy the body.
         await blobStreamResponse.Value.Content.CopyToAsync(Response.Body).ConfigureAwait(false);
 
         // All done.