Add default value to doc_type in index method as it is by default set…

… to '_doc',

as while mocking with es 7.x it fails if we do not add the doc type specifically
when calling the index method with the index mock to accept doc_type by default
to _doc as with elastic search 7.x the value is set to _doc by default,
and when mocking the index method we do have to pass the doc_type as mandatory

elasticmock/fake_elasticsearch.py

@@ -44,7 +44,7 @@ def info(self, params=None):
 
     @query_params('consistency', 'op_type', 'parent', 'refresh', 'replication',
                   'routing', 'timeout', 'timestamp', 'ttl', 'version', 'version_type')
-    def index(self, index, doc_type, body, id=None, params=None):
+    def index(self, index, body, doc_type='_doc', id=None, params=None):
         if index not in self.__documents_dict:
             self.__documents_dict[index] = list()