r.content is None of there's an invalid response.

Fixes #236

requests/models.py

@@ -496,7 +496,11 @@ class Response(object):
                                'already consumed')
 
         # Read the contents.
-        self._content = self.raw.read()
+        try:
+            self._content = self.raw.read()
+        except AttributeError:
+            return None
+
 
         # Decode GZip'd content.
         if 'gzip' in self.headers.get('content-encoding', ''):

test_requests.py

@@ -529,6 +529,11 @@ class RequestsTestSuite(unittest.TestCase):
         assert params3['b'] in r3.content
         assert params3['c'] in r3.content
 
+    def test_invalid_content(self):
+
+        r = requests.get('http://somedomainthatclearlydoesntexistg.com')
+        assert r.content == None
+
 
 if __name__ == '__main__':
     unittest.main()