Currently the unique package tries to clone strings that get stored in
its internal map to avoid retaining large strings.
However, this falls over entirely due to the fact that the original
string is *still* stored in the map as a key. Whoops. Fix this by
storing the cloned value in the map instead.
This change also adds a test which fails without this change.
Change-Id: I1a6bb68ed79b869ea12ab6be061a5ae4b4377ddb
Reviewed-on: https://go-review.googlesource.com/c/go/+/610738 Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>