Merge #10019: Fix regression relative to 2.6: add newlines if indent=0

Patch by Amaury Forgeot d'Arc, updated by Sando Tosi.

Lib/test/json_tests/test_indent.py

@@ -2,6 +2,7 @@ from unittest import TestCase
 
 import json
 import textwrap
+from io import StringIO
 
 class TestIndent(TestCase):
     def test_indent(self):
@@ -43,3 +44,18 @@ class TestIndent(TestCase):
         self.assertEqual(h3, h)
         self.assertEqual(d2, expect.expandtabs(2))
         self.assertEqual(d3, expect)
+
+    def test_indent0(self):
+        h = {3: 1}
+        def check(indent, expected):
+            d1 = json.dumps(h, indent=indent)
+            self.assertEqual(d1, expected)
+
+            sio = StringIO()
+            json.dump(h, sio, indent=indent)
+            self.assertEqual(sio.getvalue(), expected)
+
+        # indent=0 should emit newlines
+        check(0, '{\n"3": 1\n}')
+        # indent=None is more compact
+        check(None, '{"3": 1}')