CSHARP-3962: Fix X509CertificateCollectionEqualityComparer comparing logic. (#691)

Author: DmitryLukyanov

src/MongoDB.Driver/SslSettings.cs

@@ -269,7 +269,11 @@ private class X509CertificateCollectionEqualityComparer : IEqualityComparer<X509
         {
             public bool Equals(X509CertificateCollection lhs, X509CertificateCollection rhs)
             {
-                if (lhs == null) { return rhs == null; }
+                if (lhs == null || rhs == null)
+                {
+                    return rhs == lhs;
+                }
+
                 return ((IEnumerable)lhs).Cast<X509Certificate>().SequenceEqual(((IEnumerable)rhs).Cast<X509Certificate>());
             }
 

tests/MongoDB.Driver.Tests/SslSettingsTests.cs

@@ -137,6 +137,7 @@ public void TestEquals()
             var certificateFileName = GetTestCertificateFileName();
             clone.ClientCertificates = new[] { new X509Certificate2(certificateFileName, "password") };
             Assert.NotEqual(settings, clone);
+            Assert.NotEqual(clone, settings);
 
             clone = settings.Clone();
             clone.ClientCertificateSelectionCallback = ClientCertificateSelectionCallback;