CSHARP-760 - default ExternalEvidence credential when credential created with username only

Author: Sridhar Nanjundeswaran

MongoDB.Driver/MongoCredential.cs

@@ -163,7 +163,7 @@ public static MongoCredential CreateGssapiCredential(string username)
             return FromComponents("GSSAPI",
                 "$external",
                 username,
-                (PasswordEvidence)null);
+                new ExternalEvidence());
         }
 
         /// <summary>

MongoDB.DriverUnitTests/MongoCredentialTests.cs

@@ -69,5 +69,17 @@ public void TestPassword()
             Assert.AreEqual("password", credentials.Password);
 #pragma warning restore
         }
+
+        [Test]
+        public void TestCreateGssapiCredentialWithOnlyUsername()
+        {
+            var username = "testuser";
+            var credential = MongoCredential.CreateGssapiCredential(username);
+            Assert.AreEqual(username, credential.Username);
+            Assert.IsInstanceOf<ExternalEvidence>(credential.Evidence);
+            Assert.AreEqual("GSSAPI", credential.Mechanism);
+            Assert.AreEqual("$external", credential.Source);
+            Assert.AreEqual(new ExternalEvidence(), credential.Evidence);
+        }
     }
 }