Update usage of deprecated method

Author: Hugo Osvaldo Barrera

vdirsyncer/storage/dav.py

@@ -124,7 +124,7 @@ def _merge_xml(items):
         return None
     rv = items[0]
     for item in items[1:]:
-        rv.extend(item.getiterator())
+        rv.extend(item.iter())
     return rv