Fix matching updates (#26)

[brainiac-five]

• fix: matching updates (Save Error after Repeat Put #26)

• add: matching KVS update tests

Explanation of the Fix

An update that is an exact key-and-value match with an existing node should return that existing node rather than nil, because the return value from update() is (across some nesting update functions across class layers) expected to be the new node (that in this special case is an old node) and assigned to pm.n in SwarmPot.Update() in mode.go

proximity-order-trie/pkg/elements/mode.go

Line 132 in 4c39490

pm.n = update

and to root in Index.Update() in index.go

proximity-order-trie/index.go

Line 101 in 4c39490

root = update

Imperfection of this PR

In hindsight this PR should probably better add tests to index_test.go instead of kvs_test.go but the tests added to kvs_test.go track how the bug was found (and reported in #26 / branch issue/26); and because of where the fix is (in pkg/elements/ops.go), the issue would appear to get tested equally well for regression in either suite.

Regarding Added Tests

Three of the tests that are being added by this PR were failing without the fix. See branch issue/26. But not all of them, because they were made to help pinpoint the exact picture of the bug, including what works although it looks similar to the failing scenarios. See issue #26.

[lexonian]

Force push fixed test prompts (deleted " - ok" / " - returns error" from t.Run calls).

[brainiac-five]

Closing because PR #30 solves it, together with issue #29.