[api] Improve robustness of TokenAuthentication

[nemesifier]

Reserved for GCI

At this line: https://github.com/openwisp/django-freeradius/blob/master/django_freeradius/api/views.py#L29

The code would fail if the authorization header does not contain any space. Proceed as follows:

• replicate the issue in a test case (this is also called Test Driven Development)
• once the test fails, fix the code so it doesn't fail