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.
Merge remote-tracking branch 'origin/master' into NEXUS-6701-archiva-blocked
NEXUS-6701: UT
Merge pull request #640 from sonatype/NEXUS-6701-archiva-blocked

REVIEW NEXUS-6701: Dont waste cycles if not needed