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.
NEXUS-6701: UT

Conflicts:
	components/nexus-core/src/test/java/org/sonatype/nexus/proxy/storage/remote/httpclient/HttpClientRemoteStorageTest.java
NEXUS-6701: Ironing out API changes from master

as cherry-picking brought them in
Merge pull request #641 from sonatype/nexus-2.9.x-NEXUS-6701-archiva-blocked

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