DATAMONGO-807 - findAndModify does not retain Class information

Using findAndUpdate did not retain _class information when used to update a whole nested type instead of single fields within the type.
UpdateMapper will check for this kind of update and add the required information to the update if the type cannot be automatically be converted.
We now use the UpdateMapper for the Update-Object in MongoTemplate#findAndModify().

Original pull request #110