Empty content in a request with python-requests

I have bee trying to get the html code of this page. The content of the page is empty but the page is functional ( i can see it in the browser)

import requests
print 'Requests version: ', requests.__version__
r = requests.get('http://ift.tt/29rzKdC', allow_redirects=True, headers={'User-Agent': 'test'})
print 'Status code: ', r.status_code
print 'Content len: ', len(r.content)

The result is

Requests version:  2.13.0
Status code:  200
Content len:  0

What can be the cause?

Thank you.