Supported transition from leader to standby according to #224

Author: sakno

src/DotNext.Tests/Net/Cluster/Consensus/Raft/Http/RaftHttpClusterTests.cs

@@ -448,7 +448,6 @@ public static async Task StandbyMode()
         await GetLocalClusterView(host3).Readiness.WaitAsync(DefaultTimeout);
 
         // suspend two nodes
-        False(await GetLocalClusterView(host1).EnableStandbyModeAsync());
         True(await GetLocalClusterView(host2).EnableStandbyModeAsync());
         True(GetLocalClusterView(host2).Standby);
         True(await GetLocalClusterView(host3).EnableStandbyModeAsync());

src/cluster/DotNext.Net.Cluster/Net/Cluster/Consensus/Raft/RaftCluster.cs

@@ -388,7 +388,7 @@ public async ValueTask<bool> EnableStandbyModeAsync(CancellationToken token = de
         ObjectDisposedException.ThrowIf(IsDisposed, this);
 
         RaftState<TMember> currentState;
-        if ((currentState = state) is FollowerState<TMember> or CandidateState<TMember>)
+        if ((currentState = state) is not StandbyState<TMember>)
         {
             var tokenSource = token.LinkTo(LifecycleToken);
             var lockTaken = false;