Backport PR #1903: Allow websocket connections from scripts

scripts don't set origin on connection

we allow these on API requests, websockets should match.

cc  rgbkrk: is there a reason we block 'missing origin' websockets when we allow the same for API requests? Is it possible for a request from a sensible browser to be missing Host and/or Origin?

notebook/base/zmqhandlers.py

@@ -132,15 +132,11 @@ class WebSocketMixin(object):
         host = self.request.headers.get("Host")
         if origin is None:
             origin = self.get_origin()
-        
-        # If no header is provided, assume we can't verify origin
-        if origin is None:
-            self.log.warn("Missing Origin header, rejecting WebSocket connection.")
-            return False
-        if host is None:
-            self.log.warn("Missing Host header, rejecting WebSocket connection.")
-            return False
-        
+
+        # If no origin or host header is provided, assume from script
+        if origin is None or host is None:
+            return True
+
         origin = origin.lower()
         origin_host = urlparse(origin).netloc