Corrected sortable behaviour on inserting new objects

I do not know if this is the way to do it. However this fixes #1086 by readding `+1` if it is an insert operation (removed [here](https://github.com/Atlantic18/DoctrineExtensions/pull/802/files#diff-5cdefb40003e6b51c3b0568760b24214R231)). If it is a plane update operation, it won't add the `+1`, just as it was done in #802.