REVIEW [2.9] NEXUS-6701: Dont waste cycles if not needed

To convert response last-modified date if not interested
in it. This also fixes Archiva remote availability detection.

Also, update DateUtils that was deprecated in HC4 to the
proper one.

Issue
https://issues.sonatype.org/browse/NEXUS-6701

CI
http://bamboo.s/browse/NX-OSSF169