Fix hostname matching with user-provided certs

[lpsinger]

User-provided certs would cause XMLStream.start_tls to fail because the SSLContext object had check_hostname set, but no server_hostname was provided to SSLContext.wrap_socket. This would cause the connetion to fail and SleekXMPP to produce the following error message:

ERROR:sleekxmpp.basexmpp:check_hostname requires server_hostname
Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/sleekxmpp/xmlstream/xmlstream.py", line 1568, in _process
    if not self.__read_xml():
  File "/usr/lib/python3.7/site-packages/sleekxmpp/xmlstream/xmlstream.py", line 1640, in __read_xml
    self.__spawn_event(xml)
  File "/usr/lib/python3.7/site-packages/sleekxmpp/xmlstream/xmlstream.py", line 1708, in __spawn_event
    handler.prerun(stanza_copy)
  File "/usr/lib/python3.7/site-packages/sleekxmpp/xmlstream/handler/callback.py", line 64, in prerun
    self.run(payload, True)
  File "/usr/lib/python3.7/site-packages/sleekxmpp/xmlstream/handler/callback.py", line 76, in run
    self._pointer(payload)
  File "/usr/lib/python3.7/site-packages/sleekxmpp/features/feature_starttls/starttls.py", line 64, in _handle_starttls_proceed
    if self.xmpp.start_tls():
  File "/usr/lib/python3.7/site-packages/sleekxmpp/xmlstream/xmlstream.py", line 940, in start_tls
    ssl_socket = self._create_secure_socket()
  File "/usr/lib/python3.7/site-packages/sleekxmpp/xmlstream/xmlstream.py", line 520, in _create_secure_socket
    return ctx.wrap_socket(self.socket, do_handshake_on_connect=False)
  File "/usr/lib/python3.7/ssl.py", line 412, in wrap_socket
    session=session
  File "/usr/lib/python3.7/ssl.py", line 810, in _create
    raise ValueError("check_hostname requires server_hostname")
ValueError: check_hostname requires server_hostname

Fix this by passing the hostname, which fortunately had been saved in XMLStream._expected_server_name.

[lpsinger]

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