Fixed #20472: response.content should be bytes on both Python 2 and 3

Without the patch having ``ConditionalGetMiddleware`` enabled under Python 3.x results in the following exception when running ``manage.py runserver``:

```
Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/wsgiref/handlers.py", line 138, in run
    self.finish_response()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/wsgiref/handlers.py", line 178, in finish_response
    for data in self.result:
  File "/Users/ambv/Documents/Projekty/Python/django/django/http/response.py", line 288, in __next__
    return self.make_bytes(next(self._iterator))
  File "/Users/ambv/Documents/Projekty/Python/django/django/http/response.py", line 270, in make_bytes
    return bytes(value)
TypeError: string argument without an encoding
```