Fix SchemaDto deserialization

[0xced]

Commit 9f5da1d (use manual serialization (#331)) does not properly handle the fact that before manual serialization hangfire.schema._id was stored as a string of the corresponding MongoSchema enum value.

With an existing hangfire.schema collection, the migration would fail with the following error:

System.InvalidCastException: Unable to cast object of type 'MongoDB.Bson.BsonString' to type 'MongoDB.Bson.BsonInt32'.
   at MongoDB.Bson.BsonValue.get_AsInt32()
   at Hangfire.Mongo.Dto.SchemaDto..ctor(BsonDocument doc)
   at lambda_method81(Closure , BsonDocument )
   at MongoDB.Bson.Serialization.Serializers.ProjectingDeserializer`2.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
   at MongoDB.Bson.Serialization.IBsonSerializerExtensions.Deserialize[TValue](IBsonSerializer`1 serializer, BsonDeserializationContext context)
   at MongoDB.Driver.Core.Operations.CursorBatchDeserializationHelper.DeserializeBatch[TDocument](RawBsonArray batch, IBsonSerializer`1 documentSerializer, MessageEncoderSettings messageEncoderSettings)
   at MongoDB.Driver.Core.Operations.FindOperation`1.CreateFirstCursorBatch(BsonDocument cursorDocument)
   at MongoDB.Driver.Core.Operations.FindOperation`1.CreateCursor(IChannelSourceHandle channelSource, IChannelHandle channel, BsonDocument commandResult)
   at MongoDB.Driver.Core.Operations.FindOperation`1.Execute(RetryableReadContext context, CancellationToken cancellationToken)
   at MongoDB.Driver.Core.Operations.FindOperation`1.Execute(IReadBinding binding, CancellationToken cancellationToken)
   at MongoDB.Driver.OperationExecutor.ExecuteReadOperation[TResult](IReadBinding binding, IReadOperation`1 operation, CancellationToken cancellationToken)
   at MongoDB.Driver.MongoCollectionImpl`1.ExecuteReadOperation[TResult](IClientSessionHandle session, IReadOperation`1 operation, ReadPreference readPreference, CancellationToken cancellationToken)
   at MongoDB.Driver.MongoCollectionImpl`1.ExecuteReadOperation[TResult](IClientSessionHandle session, IReadOperation`1 operation, CancellationToken cancellationToken)
   at MongoDB.Driver.MongoCollectionImpl`1.FindSync[TProjection](IClientSessionHandle session, FilterDefinition`1 filter, FindOptions`2 options, CancellationToken cancellationToken)
   at MongoDB.Driver.MongoCollectionImpl`1.<>c__DisplayClass46_0`1.<FindSync>b__0(IClientSessionHandle session)
   at MongoDB.Driver.MongoCollectionImpl`1.UsingImplicitSession[TResult](Func`2 func, CancellationToken cancellationToken)
   at MongoDB.Driver.MongoCollectionImpl`1.FindSync[TProjection](FilterDefinition`1 filter, FindOptions`2 options, CancellationToken cancellationToken)
   at MongoDB.Driver.FindFluent`2.ToCursor(CancellationToken cancellationToken)
   at MongoDB.Driver.IAsyncCursorSourceExtensions.FirstOrDefault[TDocument](IAsyncCursorSource`1 source, CancellationToken cancellationToken)
   at MongoDB.Driver.IFindFluentExtensions.FirstOrDefault[TDocument,TProjection](IFindFluent`2 find, CancellationToken cancellationToken)
   at Hangfire.Mongo.Migration.MongoMigrationManager.GetCurrentSchema(IMongoDatabase database)
   at Hangfire.Mongo.Migration.MongoMigrationManager.Migrate(MongoBackupStrategy backupStrategy, MongoMigrationStrategy migrationStrategy)
   at Hangfire.Mongo.Migration.MongoMigrationManager.MigrateIfNeeded(MongoStorageOptions storageOptions, IMongoDatabase database)
   at Hangfire.Mongo.MongoStorage..ctor(IMongoClient mongoClient, String databaseName, MongoStorageOptions storageOptions)
   at Hangfire.Mongo.MongoStorage..ctor(MongoClientSettings mongoClientSettings, String databaseName, MongoStorageOptions storageOptions)

[admjlg]

Thanks!

[0xced]

Thanks for merging and releasing a new version so quickly! 🥳