DNN-5206: DotNetNuke.Services.FileSystemFileManager.WriteFileToHttpContext(): Do not send negative "Content-Length" header

This pull request fixes the problem, that files > 2GB are not downloadable using the with LinkClick.aspx. 

Please note: The problem, that the file size are negative for large files (due too integer overflow) is not fixed in the pull request. This requires changing the API (see @cathalconnolly comments here https://dnntracker.atlassian.net/browse/DNN-5206)