DATAMONGO-852 - Update keeps track of fields to be modified.

Update holds a set of fields that modifications are registered for. This information is used to determine if a modification is registered for the version field of a versioned entity. The change was introduced since the present solution did not correctly find the version property correctly within the DBObject resulting from the mapped update.

In case version property is already included in Update automatic version update via $inc is will be skipped.

Original pull request: #126