fixes #115, sets original file canonical path on cache if present

This PR fixes #115.  The code was blowing up as it was looking for a `canonicalPath` on an `AssetFile` when I think it was actually looking for the `canonicalPath` on the original file.

I also changed the parameter name to `originalFileName` as I thought that better showed the intent of the variable and didn't see any other uses that it contradicted.

It also adds a test for the cache when the optional parameter is set and ensures that it clears the cache on a cache miss.