Make F() nad Q() neutral elements

Fixes #166 Thanks mdj2 for the report!

elasticsearch_dsl/filter.py

@@ -1,6 +1,6 @@
 from .utils import DslBase, BoolMixin, _make_dsl_class
 
-def F(name_or_filter, filters=None, **params):
+def F(name_or_filter='match_all', filters=None, **params):
     # 'and/or', [F(), F()]
     if filters is not None:
         # someone passed in positional argument to F outside of and/or or query

elasticsearch_dsl/query.py

@@ -1,7 +1,7 @@
 from .utils import DslBase, BoolMixin, _make_dsl_class
 from .function import SF, ScoreFunction
 
-def Q(name_or_query, **params):
+def Q(name_or_query='match_all', **params):
     # {"match": {"title": "python"}}
     if isinstance(name_or_query, dict):
         if params:

test_elasticsearch_dsl/test_filter.py

@@ -2,6 +2,12 @@
 
 from pytest import raises
 
+def test_empty_F_is_match_all():
+    f = filter.F()
+
+    assert isinstance(f, filter.MatchAll)
+    assert filter.MatchAll() == f
+
 def test_and_can_be_created_with_a_list():
     f = filter.F('and', [filter.F('term', field='value')])
 

test_elasticsearch_dsl/test_query.py

@@ -2,6 +2,12 @@
 
 from pytest import raises
 
+def test_empty_Q_is_match_all():
+    q = query.Q()
+
+    assert isinstance(q, query.MatchAll)
+    assert query.MatchAll() == q
+
 def test_match_to_dict():
     assert {"match": {"f": "value"}} == query.Match(f='value').to_dict()