Fix refresh cookie token bugs

[mehdithreem]

There was a bug in which /token/refresh/ did not read and update the refresh token cookie. A small refactoring is also included.

[iMerica]

Thanks for submitting this pull request. I'm setting up Circle CI to build this, but I noticed the test dj_rest_auth.tests.test_api.APIBasicTests.test_custom_token_refresh_view is failing locally. I'm not sure if it's because the assertion is no longer valid or your changes introduced a genuine regression.

Can you have a look? You can run tests locally using tox --parallel (run all tests in all supported Django/Python versions -- in parallel).

[mehdithreem]

Sure, I will check this.

[mehdithreem]

@iMerica all tests passed.

[iMerica]

LGTM 👍