Not able to stub cookies for normal (non-directed requests)

[mjholgate]

Please see the new unit test I've added here:

https://github.com/mjholgate/Nocilla/tree/set-cookie-unit-test

It fails, but I think it should pass (i.e. the cookie should be set, and should be able to be read back from the cookie store).

Weirdly, the existing unit test which does the same thing for redirected requests (http status code 301), works fine.

[mjholgate]

Added a fix for the bug to the pull request.