Avoid IE Brokenness When using Vary and Attachments

Apparently Internet Explorer (6 and 7) have a bug whereby if you blindly attach a PDF or some other file, it will choke. The problem lies in the Vary header (bug described in http://support.microsoft.com/kb/824847/en-us?spid=8722&sid=global). To use, just add to the beginning of your middleware classes.