Fix #1370 by making copy of default AuthMechanisms

Author: evolvedlight

projects/RabbitMQ.Client/client/api/ConnectionFactory.cs

@@ -153,7 +153,7 @@ public sealed class ConnectionFactory : ConnectionFactoryBase, IAsyncConnectionF
         /// <summary>
         ///  SASL auth mechanisms to use.
         /// </summary>
-        public IList<IAuthMechanismFactory> AuthMechanisms { get; set; } = DefaultAuthMechanisms;
+        public IList<IAuthMechanismFactory> AuthMechanisms { get; set; } = DefaultAuthMechanisms.ToList();
 
         /// <summary>
         /// Address family used by default.

projects/Unit/TestConnectionFactory.cs

@@ -279,5 +279,14 @@ public void TestCreateConnectioUsesValidEndpointWhenMultipleSupplied()
             var ep = new AmqpTcpEndpoint("localhost");
             using(IConnection conn = cf.CreateConnection(new List<AmqpTcpEndpoint> { invalidEp, ep })) {};
         }
+
+        [Test]
+        public void TestCreateConnectionTwiceDoesntClearAuthMechanisms()
+        {
+            var cf = new ConnectionFactory();
+            cf.AuthMechanisms.Clear();
+            var cf2 = new ConnectionFactory();
+            Assert.That(cf2.AuthMechanisms.Count, Is.EqualTo(1));
+        }
     }
 }