require explicit plotly json encoding by objects

`to_json` was actually used by pandas, and was called by our json encoder before the correct pandas series json encoder was called

Author: chriddyp

plotly/grid_objs/grid_objs.py

@@ -35,7 +35,7 @@ def __str__(self):
     def __repr__(self):
         return 'Column("{}", {})'.format(self.data, self.name)
 
-    def to_json(self):
+    def to_plotly_json(self):
         return {'name': self.name, 'data': self.data}
 
 

plotly/utils.py

@@ -128,7 +128,7 @@ def sageJSONEncoder(self, obj):
 
     def builtinJSONEncoder(self, obj):
         try:
-            return obj.to_json()
+            return obj.to_plotly_json()
         except AttributeError:
             return None