core, plugins: fixed bad use of children instead of elements() for domish.Element instances.

src/plugins/plugin_xep_0020.py

@@ -55,7 +55,7 @@ class XEP_0020(object):
         """Check element's children to find feature elements
         @param elt: domish.Element
         @return: feature elements"""
-        return filter(lambda elt: elt.name == 'feature', elt.elements())
+        return [child for child in elt.elements() if child.name == 'feature']
 
     def getChoosedOptions(self, elt):
         """Return choosed feature for feature element