[postprocessor:metadata] filter private entries

i.e. keys starting with an underscore

test/test_postprocessor.py

@@ -151,6 +151,9 @@ class MetadataTest(BasePostprocessorTest):
             "ascii"    : True,
             "indent"   : 2,
             "extension": "JSON",
+        }, {
+            "public"   : "hello",
+            "_private" : "world",
         })
 
         self.assertEqual(pp.write    , pp._write_json)
@@ -167,7 +170,8 @@ class MetadataTest(BasePostprocessorTest):
         self.assertEqual(self._output(m), """{
   "category": "test",
   "extension": "ext",
-  "filename": "file"
+  "filename": "file",
+  "public": "hello"
 }
 """)
 

test/test_results.py

@@ -207,7 +207,7 @@ class ResultJob(job.DownloadJob):
     def _update_kwdict(self, kwdict, to_list=True):
         if to_list:
             self.kwdict_list.append(kwdict.copy())
-        kwdict = self._filter(kwdict)
+        kwdict = util.filter_dict(kwdict)
         self.kwdict_hash.update(
             json.dumps(kwdict, sort_keys=True, default=str).encode())
 

test/test_util.py

@@ -358,6 +358,21 @@ class TestOther(unittest.TestCase):
         self.assertEqual(
             d, {1: 123, 2: 123, 3: 0, 4: {11: 321, 12: 321, 13: 0}})
 
+    def test_filter_dict(self):
+        d = {}
+        r = util.filter_dict(d)
+        self.assertEqual(r, d)
+        self.assertIsNot(r, d)
+
+        d = {"foo": 123, "bar": [], "baz": None}
+        r = util.filter_dict(d)
+        self.assertEqual(r, d)
+        self.assertIsNot(r, d)
+
+        d = {"foo": 123, "_bar": [], "__baz__": None}
+        r = util.filter_dict(d)
+        self.assertEqual(r, {"foo": 123})
+
     def test_number_to_string(self, f=util.number_to_string):
         self.assertEqual(f(1)     , "1")
         self.assertEqual(f(1.0)   , "1.0")