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BugFix: TinyCache CanOperate fails if VirtualCacheFile is null

I have reversed the logic in CanOperate as the HasFileIOPermisions
requires a VirtualCacheFile.

I have also put wrappers around HostingEnvironment to facilitate
testing.

I have added a check for CanProcess eventArgs being null. We get a more
sensible excetion that way.
-GravitasExtra