REVIEW NEXUS-6838: "Rude" fix, adding SNI to TLS/HTTPS

This is the "rude" way as described in HTTPCLIENT-1119.

Other solution is described in other comment, in short: create
"plain" socket, and then wrap that socket with SSLSocket, but
for that to work you'd need to have host dedicated SSLContext
(for each HOST you access).

SNI works only on Java7+

Forward port of
https://github.com/sonatype/nexus-oss/pull/714

Issue:
https://issues.sonatype.org/browse/NEXUS-6838

CI
http://bamboo.s/browse/NX-OSSF253-1 :white_check_mark: