need to allow the GetServer API to expose KEYS/SCAN, which is supported

src/StackExchange.Redis/ConnectionMultiplexer.cs

@@ -1333,7 +1333,6 @@ public IServer GetServer(EndPoint endpoint, object asyncState = null)
             if (endpoint == null) throw new ArgumentNullException(nameof(endpoint));
             switch (RawConfig.Proxy)
             {
-                case Proxy.RedisClusterProxy:
                 case Proxy.Twemproxy:
                     throw new NotSupportedException($"The server API is not available via {RawConfig.Proxy}");
             }

tests/StackExchange.Redis.Tests/RedisClusterProxyTests.cs

@@ -23,10 +23,11 @@ public void CanConnectToClusterProxy()
                 Assert.Equal(expected, actual);
 
                 // check it knows that we're dealing with a cluster
-                var ex = Assert.Throws<NotSupportedException>(() => conn.GetServer("abc"));
-                Assert.Equal("The server API is not available via RedisClusterProxy", ex.Message);
+                var server = conn.GetServer(conn.GetEndPoints()[0]);
+                Assert.Equal(ServerType.RedisClusterProxy, server.ServerType);
+                _  = server.Echo("abc");
 
-                ex = Assert.Throws<NotSupportedException>(() => conn.GetSubscriber("abc"));
+                var ex = Assert.Throws<NotSupportedException>(() => conn.GetSubscriber("abc"));
                 Assert.Equal("The pub/sub API is not available via RedisClusterProxy", ex.Message);
 
                 // test a script