Issue with filepath for keyfile.

[omkarpg0931]

Hi,

I was testing your module for my project on Linux with Node.js v8.13.
The concatenated path string for path.resolve functionality in "fetchCachedCertificate" function in file authentication/util/Cache.js does not work on Linux as the filepath is created with "\", which is supported on Windows only.
On Linux, "/" is supported by path.