credential expiration comes back as numeric Inf (unclassed)

[tyner]

We had a user set only these three environment variables:

AWS_ACCESS_KEY_ID AWS_SECRET_ACCESS_KEY AWS_SESSION_TOKEN

and then when paws.common::locate_credentials() was called, its return had expiration equal to numeric Inf (with no class attribute; usually it would have POSIXct). I not sure whether Inf is a valid expiration, but if it is, should it be classed? Conversely, if it is not a valid expiration, should an error/warning be thrown?

The use case is if someone is caching the credentials and needs to figure out if the cached credentials are expired.