Dont set Content-Length Header Attribute for the FileDownloadTestsite

Currently, the Content-Length Header is set when Files are downloaded in Development.WebTesting.TestSite.FileDownloadTest and Development.WebTesting.TestSite.FileDownloadHandler.

This leads to failed Download Tests if a Hosting Method is used which automatically compresses Files on download, as the Content-Length of the downloaded file is now less than reported in the header and the file reports as being downloaded incomplete.