Change how ssl_wrap_socket handles certfiles and keyfiles to match the r...

...egular python ssl library

I first noticed this when I started getting my requests rejected by the server when I started using pyopenssl. I had been using requests and had been passing a client certificate in like `requests.get(..., cert='foo.pem')`

When using the regular ssl library, we call ssl.wrap_socket, which uses the certfile as both the certfile and the keyfile if the keyfile is None [1].

When using pyopenssl, we should keep the same expectation.

[1] https://github.com/python/cpython/blob/455a24625d461ea841ea42b277aa0fb72065a04a/Lib/ssl.py#L491-L492