Make the widget property_lock store the JSON state, not the original object

Comparison between arbitrary python objects can get crazy, whereas we really
only care about if the JSON state we are syncing is different.  So with this change,
we store and check against the JSON state.

IPython/html/widgets/widget.py

@@ -202,9 +202,7 b' class Widget(LoggingConfigurable):'
             raise ValueError("key must be a string, an iterable of keys, or None")
         state = {}
         for k in keys:
-            f = self.trait_metadata(k, 'to_json')
-            if f is None:
-                f = self._trait_to_json
+            f = self.trait_metadata(k, 'to_json', self._trait_to_json)
             value = getattr(self, k)
             state[k] = f(value)
         return state
@@ -255,6 +253,8 b' class Widget(LoggingConfigurable):'
     def _lock_property(self, key, value):
         """Lock a property-value pair.
 
+        The value should be the JSON state of the property.
+
         NOTE: This, in addition to the single lock for all state changes, is 
         flawed.  In the future we may want to look into buffering state changes 
         back to the front-end."""
@@ -280,7 +280,9 b' class Widget(LoggingConfigurable):'
 
     def _should_send_property(self, key, value):
         """Check the property lock (property_lock)"""
-        if (key == self._property_lock[0] and value == self._property_lock[1]):
+        to_json = self.trait_metadata(key, 'to_json', self._trait_to_json)
+        if (key == self._property_lock[0]
+            and to_json(value) == self._property_lock[1]):
             return False
         elif self._send_state_lock > 0:
             self._states_to_send.add(key)
@@ -311,12 +313,10 b' class Widget(LoggingConfigurable):'
         """Called when a state is received from the front-end."""
         for name in self.keys:
             if name in sync_data:
-                f = self.trait_metadata(name, 'from_json')
-                if f is None:
-                    f = self._trait_from_json
-                value = f(sync_data[name])
-                with self._lock_property(name, value):
-                    setattr(self, name, value)
+                json_value = sync_data[name]
+                from_json = self.trait_metadata(name, 'from_json', self._trait_from_json)
+                with self._lock_property(name, json_value):
+                    setattr(self, name, from_json(json_value))
 
     def _handle_custom_msg(self, content):
         """Called when a custom msg is received."""