switch default ws_url logic to js side

In some cases (proxies, #3305), the request object doesn't have the right information about the originating information.  This changes the default behavior, so that `ws_url` is generally empty by default, which the javascript takes to mean 'the same as http'.  This is simpler and should be more resilient than trying a guess on server-side.

IPython/frontend/html/notebook/base/handlers.py

@@ -211,15 +211,10 @@ class IPythonHandler(AuthenticatedHandler):
     def ws_url(self):
         """websocket url matching the current request
 
-        turns http[s]://host[:port] into
-                ws[s]://host[:port]
+        By default, this is just `''`, indicating that it should match
+        the same host, protocol, port, etc.
         """
-        proto = self.request.protocol.replace('http', 'ws')
-        host = self.settings.get('websocket_host', '')
-        # default to config value
-        if host == '':
-            host = self.request.host # get from request
-        return "%s://%s" % (proto, host)
+        return self.settings.get('websocket_url', '')
     
     @property
     def mathjax_url(self):

IPython/frontend/html/notebook/static/services/kernels/js/kernel.js

@@ -102,7 +102,11 @@ var IPython = (function (IPython) {
         console.log("Kernel started: ", json.kernel_id);
         this.running = true;
         this.kernel_id = json.kernel_id;
-        this.ws_url = json.ws_url;
+        var ws_url = json.ws_url;
+        if (ws_url.match(/wss?:\/\//) == null) {
+            ws_url = "ws" + location.origin.substr(4) + ws_url;
+        };
+        this.ws_url = ws_url;
         this.kernel_url = this.base_url + "/" + this.kernel_id;
         this.start_channels();
         $([IPython.events]).trigger('status_started.Kernel', {kernel: this});