`ILookupGrain.AddOrUpdate` does not what the function name says

[flensrocker]

I'm playing around with Orleans and found your repository, because I was looking into managing Identities with Orleans.

I stumble across ILookupGrain.AddOrUpdate. It will never update a value, once it's in the dictionary.
If this is on purpose, shouldn't it be name just Add?

Otherwise, it should maybe do something like:

diff --git a/src/Orleans.IdentityStore/Grains/LookupGrain.cs b/src/Orleans.IdentityStore/Grains/LookupGrain.cs
index 1ef3f77..b094bb8 100644
--- a/src/Orleans.IdentityStore/Grains/LookupGrain.cs
+++ b/src/Orleans.IdentityStore/Grains/LookupGrain.cs
@@ -34,11 +34,11 @@ namespace Orleans.IdentityStore.Grains

         public async Task<bool> AddOrUpdate(string value, Guid grainKey)
         {
-            if (_index.State.Index.ContainsKey(value))
-                return false;
-
-            _index.State.Index[value] = grainKey;
-            await _index.WriteStateAsync();
+            if (!_index.State.Index.TryGetValue(value, out var oldGrainKey) || grainKey != oldGrainKey)
+            {
+                _index.State.Index[value] = grainKey;
+                await _index.WriteStateAsync();
+            }
             return true;
         }

Thank you!