StaticFileHandler: DefaultFilePath bug and large files improvement

Fix problem with DefaultFilePath: if you have a file in subfolder with the same name as the DefaultFilePath ServiceStack serves the DefaultFilePath always if the LastModified <= DefaultFileModified. Ex: "/index.html" and "/app/index.html"

WriteTo(this Stream inStream, Stream outStream) writes to outStream with
a 4Kb buffer. Downloading large files(+1Mb) is very slowly because is
sending chunks of 4kb to the client. Writing with a bigger buffer
increases the performance.