[es6] remove `_all` field in search_helpers

this magic field does not exist anymore with elasticsearch 6
use computed field which it is named `alltext` by convention

cubicweb_elasticsearch/search_helpers.py

@@ -37,7 +37,7 @@ def compose_search(search, query=None, fields=(),
     query:
         text of the query to be composed (can contain quotes)
     fields:
-        restrict and boost search on certain fields eg. ('title^2', '_all')
+        restrict and boost search on certain fields eg. ('title^2', 'alltext')
     fuzzy:
         add a fuzzy search element to part of the query generated
         https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-fuzzy-query.html
@@ -73,12 +73,12 @@ def compose_search(search, query=None, fields=(),
     # highfrequency/lowfrequency query
     # https://www.elastic.co/blog/stop-stopping-stop-words-a-look-at-common-terms-query
     if common:
-        common_query = dsl_query.Common(_all={"query": query,
-                                              "cutoff_frequency": cutoff_frequency,
-                                              "low_freq_operator": "and",
-                                              "minimum_should_match": {
-                                                  "high_freq": "70%"
-                                              }})
+        common_query = dsl_query.Common(alltext={"query": query,
+                                                 "cutoff_frequency": cutoff_frequency,
+                                                 "low_freq_operator": "and",
+                                                 "minimum_should_match": {
+                                                     "high_freq": "70%"
+                                                 }})
         should.append(common_query)
 
     elements = query.split()
@@ -97,7 +97,7 @@ def compose_search(search, query=None, fields=(),
 
         else:
             if fuzzy:
-                should.append(dsl_query.Fuzzy(_all=element))
+                should.append(dsl_query.Fuzzy(alltext=element))
     bool_query = dsl_query.Bool(must=must,
                                 must_not=must_not,
                                 should=should,