Make Response.content return None if raw is None

Add test.

requests/models.py

@@ -769,7 +769,7 @@ class Response(object):
                 raise RuntimeError(
                     'The content for this response was already consumed')
 
-            if not self.status_code:
+            if self.status_code == 0 or self.raw is None:
                 self._content = None
             else:
                 self._content = bytes().join(self.iter_content(CONTENT_CHUNK_SIZE)) or bytes()

tests/test_requests.py

@@ -1094,6 +1094,10 @@ class TestRequests:
         total_seconds = ((td.microseconds + (td.seconds + td.days * 24 * 3600) * 10**6) / 10**6)
         assert total_seconds > 0.0
 
+    def test_empty_response_has_content_none(self):
+        r = requests.Response()
+        assert r.content is None
+
     def test_response_is_iterable(self):
         r = requests.Response()
         io = StringIO.StringIO('abc')