In async TLS do not ignore a ssl_context given as an argument [#951].

In async TLS do not ignore a ssl_context given as an argument [#951].

The async TLS code would always fail if given an ssl_context instead
of making one, as it set the passed paramter to None and then called
into the async socket backend, which would make a regular TCP socket
(i.e. no TLS), which would be rejected by the server as it wasn't
using TLS.