GetAsync with DataRetriever doesn't allow to set TimeSpan.Zero to expiration

[jkatsiotis]

I believe expiration parameter should be a nullable TimeSpan.

How does that sound?

[catcherwong]

@jkatsiotis Thanks for your interest in this project.

In v1.x, the expiration was designed a not nullable parameter, all cached values should be hot cache, not cold cache.

And this feature will in v2.x. (#470)