Only use username from ssh config as default

If a username is specified in the `Login` keyword, it should supersede
the username from the ssh config.

src/SSHLibrary/client.py

@@ -882,8 +882,10 @@ def _login(self, username, password, allow_agent=False, look_for_keys=False, pro
                read_config=False, jumphost_connection=None, keep_alive_interval=None):
         if read_config:
             hostname = self.config.host
-            self.config.host, username, self.config.port, proxy_cmd = \
+            self.config.host, username_from_config, self.config.port, proxy_cmd = \
                 self._read_login_ssh_config(hostname, username, self.config.port, proxy_cmd)
+            if not username or username == b'None':
+                username = username_from_config
 
         sock_tunnel = None