Fix terminals with Tornado 3

The websocket handler auth checking was calling clear_cookie(), which
threw an error because it doesn't make sense for Websockets. It doesn't
seem important, and we silence it in our other websocket handlers, so
silencing it here too.

IPython/html/terminal/handlers.py

@@ -1,44 +1,48 b''
 #encoding: utf-8
 """Tornado handlers for the terminal emulator."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 import tornado
 from tornado import web
 import terminado
 from ..base.handlers import IPythonHandler
 
 class TerminalHandler(IPythonHandler):
     """Render the terminal interface."""
     @web.authenticated
     def get(self, term_name):
         self.write(self.render_template('terminal.html',
                    ws_path="terminals/websocket/%s" % term_name))
 
 class NewTerminalHandler(IPythonHandler):
     """Redirect to a new terminal."""
     @web.authenticated
     def get(self):
         name, _ = self.application.terminal_manager.new_named_terminal()
         self.redirect(name, permanent=False)
 
 class TermSocket(terminado.TermSocket, IPythonHandler):
     def get(self, *args, **kwargs):
         if not self.get_current_user():
             raise web.HTTPError(403)
 
         # FIXME: only do super get on tornado ≥ 4
         # tornado 3 has no get, will raise 405
         if tornado.version_info >= (4,):
             return super(TermSocket, self).get(*args, **kwargs)
+    
+    def clear_cookie(self, *args, **kwargs):
+        """meaningless for websockets"""
+        pass
 
     def open(self, *args, **kwargs):
         if tornado.version_info < (4,):
             try:
                 self.get(*self.open_args, **self.open_kwargs)
             except web.HTTPError:
                 self.close()
                 raise
 
         super(TermSocket, self).open(*args, **kwargs)