NEXUS-5817: Resource fetcher should obey ProxyMode

As ResourceFetcher digs deeply (and circumvents Repository logic)
to perform remote access, the ProxyMode of repository should be
explicitly handled, to prevent remote access from repository
that should actually not go remote (is manually or auto blocked)
NEXUS-5817: Added since tag
NEXUS-5817: UT verify ResourceFetcher obeys ProxyMode
Merge pull request #99 from sonatype/nexus-5817-index-resource-fetcher-obey-proxymode

NEXUS-5817: index resource fetcher should obey proxymode