[1.9.x] Clarified that the test client response content attribute is bytes.

Backport of 484edc81c1 from master

docs/ref/request-response.txt

@@ -949,7 +949,7 @@ Typical usage could look like::
     >>> from django.http import JsonResponse
     >>> response = JsonResponse({'foo': 'bar'})
     >>> response.content
-    '{"foo": "bar"}'
+    b'{"foo": "bar"}'
 
 
 Serializing non-dictionary objects