Fix cancellation of RPC methods

Fixes #1750

* Start by adding a test that reproduces the error. Give a 5ms cancellation to `BasicConsumeAsync`, with a much longer delay via Toxiproxy. If running in debug mode, you will see the same `task canceled` exception, but it does not propagate to the test itself.

projects/Test/Integration/TestToxiproxy.cs

@@ -36,6 +36,7 @@
 using System.Threading.Tasks;
 using Integration;
 using RabbitMQ.Client;
+using RabbitMQ.Client.Events;
 using RabbitMQ.Client.Exceptions;
 using Toxiproxy.Net.Toxics;
 using Xunit;
@@ -409,6 +410,47 @@ public async Task TestPublisherConfirmationThrottling()
             Assert.Equal(TotalMessageCount, publishCount);
         }
 
+        [SkippableFact]
+        [Trait("Category", "Toxiproxy")]
+        public async Task TestBasicConsumeCancellation_GH1750()
+        {
+            Skip.IfNot(AreToxiproxyTestsEnabled, "RABBITMQ_TOXIPROXY_TESTS is not set, skipping test");
+            Assert.NotNull(_toxiproxyManager);
+            Assert.Null(_conn);
+            Assert.Null(_channel);
+
+            ConnectionFactory cf = CreateConnectionFactory();
+            cf.AutomaticRecoveryEnabled = false;
+            cf.TopologyRecoveryEnabled = false;
+
+            cf.Endpoint = new AmqpTcpEndpoint(IPAddress.Loopback.ToString(), _proxyPort);
+            _conn = await cf.CreateConnectionAsync();
+            _channel = await _conn.CreateChannelAsync();
+
+            QueueDeclareOk q = await _channel.QueueDeclareAsync();
+
+            var consumer = new AsyncEventingBasicConsumer(_channel);
+            consumer.ReceivedAsync += (o, a) =>
+            {
+                return Task.CompletedTask;
+            };
+
+            string toxicName = $"rmq-localhost-delay-{Now}-{GenerateShortUuid()}";
+            var latencyToxic = new LatencyToxic
+            {
+                Name = toxicName
+            };
+            latencyToxic.Attributes.Latency = 500;
+            latencyToxic.Toxicity = 1.0;
+            latencyToxic.Stream = ToxicDirection.DownStream;
+
+            Task<LatencyToxic> addToxicTask = _toxiproxyManager.AddToxicAsync(latencyToxic);
+            await addToxicTask.WaitAsync(WaitSpan);
+
+            using var cts = new CancellationTokenSource(5);
+            await _channel.BasicConsumeAsync(q.QueueName, true, consumer, cts.Token);
+        }
+
         private bool AreToxiproxyTestsEnabled
         {
             get