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+
NEXUS-6838: Apply animal sniffer annotation

Change is fine, locally tested, but Animal Sniffer barks on the change
as it uses JRE internal class.
NEXUS-6838: Added more comment, no code change
Merge pull request #714 from sonatype/nexus-2.10.x-NEXUS-6838-https-sni

REVIEW [2.10] NEXUS-6838: "Rude" fix, adding SNI to TLS/HTTPS