CSHARP-1879: Freeze base class map when derived class map is frozen.

Author: rstam

src/MongoDB.Bson/Serialization/BsonClassMap.cs

@@ -481,6 +481,7 @@ public BsonClassMap Freeze()
                             {
                                 _baseClassMap = LookupClassMap(baseType);
                             }
+                            _baseClassMap.Freeze();
                             _discriminatorIsRequired |= _baseClassMap._discriminatorIsRequired;
                             _hasRootClass |= (_isRootClass || _baseClassMap.HasRootClass);
                             _allMemberMaps.AddRange(_baseClassMap.AllMemberMaps);

src/MongoDB.Bson/Serialization/Serializers/BsonClassMapSerializer.cs

@@ -48,6 +48,10 @@ public BsonClassMapSerializer(BsonClassMap classMap)
                 var message = string.Format("Must be a BsonClassMap for the type {0}.", typeof(TClass));
                 throw new ArgumentException(message, "classMap");
             }
+            if (!classMap.IsFrozen)
+            {
+                throw new ArgumentException("Class map is not frozen.", nameof(classMap));
+            }
 
             _classMap = classMap;
         }