Do not update ttl on reads for absolute expirations

[krispenner]

Only update cached item ttl on reads if sliding expiration was explicitly set in cache entry options.

Closes #15

[msftclas]

All CLA requirements met.

[ealsur]

Please add an entry on the changelog.md file in the Unreleased section, under the last item of the list, which points to your PR (like the one that is already there)

[ealsur]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[ealsur]

I'll merge upon resolution of the open comment.

[ealsur]

Thanks @krispenner !