Reset backend state when deleting index

Reset setup_complete and existing_mapping when an index is
deleted. This ensures create_index is called later to restore
the settings properly.

Author: Alexey Kotlyarov

haystack/backends/elasticsearch_backend.py

@@ -218,6 +218,8 @@ def clear(self, models=[], commit=True):
         try:
             if not models:
                 self.conn.delete_index(self.index_name)
+                self.setup_complete = False
+                self.existing_mapping = {}
             else:
                 models_to_delete = []