Update to the Scriptable Web Test Plugin

I've updated my web test scriptable parameters plugin to fix a bug with scripted context parameters and extracted tests and add the ability to to script the file name property of any file upload params.

For more information see:

  https://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=3006040&SiteID=1

Many thanks to Lewis for his hard work in developing a repro and to Duat Le for spotting the casting crime I was committing.

Hope this is useful.

Neil Kidd.