Net::HTTPS should use SSLContext's defaults

Passing `nil` into `SSLContext` for `ssl_version` causes
OpenSSL to use its internal default.  For OpenSSL 1.0.1 this is
`:TLSv1_2`.

I added the defaults for the `SSLContext` options too, while
I was at it.

I found this when compiling ruby-1.9.3-p448 with OpenSSL 1.0.1.  I couldn't get gems from a local gem-server running with apache2 (SSL enabled) on CentOS 6.