issue in CLI competitions leaderboard download for stream contents

When use CLI `kaggle competitions leaderboard favorita-grocery-sales-forecasting -d` it returns stream data and it raises a key error on `size = int(response.headers['Content-Length'])` because the server doesn't return `Content-Length` in headers.

Added a pull request to handle this exception for downloading leaderboard: https://github.com/Kaggle/kaggle-api/pull/354