[DS-1132]: ItemImport BitStream Registration does not properly set the Description

Corresponding JIRA issue: https://jira.duraspace.org/browse/DS-1132

This PR fixes the issue.
However, keep in mind, that the fix is only for the case where a line of the "contents" file is in the format:

-r -r n -f filepath\t[other_options]

where n is the assetstore index.

All the other cases where handled successfully by the old code.