Adding a "content_length" option for remote uploads

The current remote upload process requires that a HEAD request be sent to the specified URL to get the "content_length" header for the video to be uploaded. To my knowledge, if you use AWS S3 and only allow files in your bucket to be accessed through pre-signed URLs, there is no way to enable HEAD requests to be sent to that URL, so the video_upload method ultimately raises an error when it expects that the content length for the video has been sent. This modification allows the content length to be passed in (optionally) as an option to the video_upload method.