Improve handling of updated items

[jarifibrahim]

When an item is updated, the existing item was updated in the store but we might return false.
Update happens here

ristretto/cache.go

Lines 229 to 231 in a093fe6

	if c.store.Update(i) {
	i.flag = itemUpdate
	}

and the false is returned here

ristretto/cache.go

Lines 232 to 239 in a093fe6

	// Attempt to send item to policy.
	select {
	case c.setBuf <- i:
	return true
	default:
	c.Metrics.add(dropSets, keyHash, 1)
	return false
	}

With this PR, we will always return true for updates, even if the policy wasn't updated.
See #167 for more details.

Fixes #167

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This change is 

[martinmr]

A couple of minor changes but it

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @anurags92, @jarifibrahim, @karlmcguire, and @manishrjain)

---

cache_test.go, line 620 at r1 (raw file):

}

func TestUpdateBug(t *testing.T) {

TestDropUpdates or something more descriptive.

---

cache_test.go, line 642 at r1 (raw file):

		}

		// This is important.

comment should say why.

[martinmr]

Reviewed 1 of 1 files at r2.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @anurags92, @jarifibrahim, @karlmcguire, and @manishrjain)