Fix json_content helper on pythons with a unicode native str type

testtools/content.py

@@ -190,9 +190,13 @@ class TracebackContent(Content):
         return length
 
 
-def json_content(data):
+def json_content(json_data):
     """Create a JSON `Content` object from JSON-encodeable data."""
-    return Content(JSON, lambda: [json.dumps(data)])
+    data = json.dumps(json_data)
+    if str_is_unicode:
+        # The json module perversely returns native str not bytes
+        data = data.encode('utf8')
+    return Content(JSON, lambda: [data])
 
 
 def text_content(text):

testtools/tests/test_content.py

@@ -155,7 +155,7 @@ class TestContent(TestCase):
 
     def test_json_content(self):
         data = {'foo': 'bar'}
-        expected = Content(JSON, lambda: [json.dumps(data)])
+        expected = Content(JSON, lambda: [_b('{"foo": "bar"}')])
         self.assertEqual(expected, json_content(data))