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`.
What is the benefit of using ssl.CERT_OPTIONAL? 
This looks like another bit of the patch which will break on Python 3.1.
While it isnt documented at  https://docs.python.org/3/library/ssl.html#ssl.SSLContext.verify_mode as being 3.2 specific, verify_mode doesnt appear in https://docs.python.org/3.1/library/ssl.html