Set XMLStream.socket after the SSL connection is made too

Fixes SCRAM-SHA-1-PLUS.

slixmpp/xmlstream/xmlstream.py

@@ -504,6 +504,7 @@ class XMLStream(asyncio.BaseProtocol):
                 der_cert = transp.get_extra_info("socket").getpeercert(True)
                 pem_cert = ssl.DER_cert_to_PEM_cert(der_cert)
                 self.event('ssl_cert', pem_cert)
+                self.socket = self.transport.get_extra_info("socket")
 
         asyncio.async(ssl_coro())