Set isDirty back to true after call to takeSnapshot()

This commit removed a line of code setting isDirty to true:
https://github.com/doctrine/mongodb-odm/commit/bd573cb7def6ab22df8c465b0d79003d26e28046
I think this line was necessary because takeSnapshot() always sets isDirty to false.  Therefore, if there have been objects added, it needs to be set to its original value.