expose urllib3's assert_hostname argument

urllib3 has the assert_hostname argument to control verification of the Common Name of the server's SSL certificate
(shazow/urllib3@2493c9b3c0ce90b82209b93cad84296244e2763a), and this was also merged to requests (@2ed976ea7147a9d0c18998e02b16d691b6798a3e).
Therefore I would like to let the user use it.