MGNLRES-31:
correct the modification date of resources after install - the date was explicitly set.

This change was reverted by MAGNOLIA-3442, which introduced:
MetaData.getModificatioDate() returns MetaData.getcreation date() if modification date is not specified.

MetaData class is currently not used by Resource and it seems that this issue is back again.
See related forum discussion.
Investigate including modification of 'concatenated multiple referenced resources' date mentioned in the forum discussion linked above.