mild Fetcher#initialize cleanup
Create persistent connection on Fetcher init.

(Might fix HTTPS request bug #3107).
Require OpenSSL instead of relying on autoload.

Apparently just letting Net::HTTP::Persistent try to autoload OpenSSL
doesn’t actually get it loaded on every machine, and then HTTPS requests
start to fail.

Fixes #3107.
Version 1.6.5 with changelog.