Use "known" label if possible when making new grid filters

tailbone/grids/core.py

@@ -159,6 +159,8 @@ class Grid(object):
         if len(args) == 1 and args[0] is None:
             self.remove_filter(key)
         else:
+            if 'label' not in kwargs and key in self.labels:
+                kwargs['label'] = self.labels[key]
             self.filters[key] = self.make_filter(key, *args, **kwargs)
 
     def remove_filter(self, key):