Fix #173: HTTPS request doesn't work when disable_ssl_certificate_validation=True

Sets `SSLContext.verify_mode` to `ssl.CERT_OPTIONAL` if `disable_ssl_certificate_validation` is `True` otherwise it is set to `ssl.CERT_REQUIRED`.
But what is wrong with the very small patch that everyone has been using for three years?
i.e. pull #289 (but it has been submitted many times previously).

I've tested it on py3.3 and 3.4 (to confirm it fixes the unittests at https://github.com/jayvdb/pywikibot-core/blob/master/tests/http_tests.py), and it has also been tested on py3.2 Windows by @fijiaaron.