Fix for compatibility with requests-2.3.0

Here's what I did to test-

1. change [these lines](https://github.com/requests/requests-ntlm/blob/master/tests/test_requests_ntlm.py#L9-L11) to contain my own NTLM server details

2. Verify without the patch, it fails under requests 2.3.0 (redacted the actual URL in case my employer minds) 
![image](https://cloud.githubusercontent.com/assets/7807926/3199467/f7cca514-ed68-11e3-8940-e1e9c07b8a48.png)

3. Test under requests 2.2.1 and 2.3.0, to verify that tests pass with both versions
![image](https://cloud.githubusercontent.com/assets/7807926/3199491/561b8ab8-ed69-11e3-93bd-58d501276e87.png)

![image](https://cloud.githubusercontent.com/assets/7807926/3199492/58dfaa72-ed69-11e3-94e4-485303b269d5.png)