CSHARP-702: fix ObjectId conversion to itself.

Author: craiggwilson

MongoDB.Bson/ObjectModel/ObjectId.cs

@@ -554,6 +554,10 @@ object IConvertible.ToType(Type conversionType, IFormatProvider provider)
                 case TypeCode.String:
                     return ((IConvertible)this).ToString(provider);
                 case TypeCode.Object:
+                    if (conversionType == typeof(ObjectId))
+                    {
+                        return this;
+                    }
                     if (conversionType == typeof(BsonObjectId))
                     {
                         return new BsonObjectId(this);

MongoDB.BsonUnitTests/ObjectModel/ObjectIdTests.cs

@@ -339,5 +339,15 @@ public void TestTryParse()
             Assert.IsFalse(ObjectId.TryParse("00102030405060708090a0b0c", out objectId1)); // too long
             Assert.IsFalse(ObjectId.TryParse(null, out objectId1)); // should return false not throw ArgumentNullException
         }
+
+        [Test]
+        public void TestConvertObjectIdToObjectId()
+        {
+            var oid = ObjectId.GenerateNewId();
+
+            var oidConverted = Convert.ChangeType(oid, typeof(ObjectId));
+
+            Assert.AreEqual(oid, oidConverted);
+        }
     }
 }