Fix: encountering expired items during the traversal process will result in zero values in the output.

[DCjanus]

No description provided.

[hashicorp-cla]

All committers have signed the CLA.

[DCjanus]

@irenarindos @mgaffney Hi, would you mind take a look at this PR? I believe it is simple enough.

[mgaffney]

Can you add a test for this?

[DCjanus]

Can you add a test for this?

Since there is a background goroutine to call deleteExpired, before we inject a custom clock, it's hard to provide such a test case.

We meet this case when: after a round of deleteExpired execution, an object expired. Before the next round of deleteExpired execution, we called the Values() method.