Add trailing_slash argument to the session

[iamanikeev]

Problem:

Django and DRF have a combination of options that makes Django either redirect with 301 or return 404 error when URL is missing a trailing slash before the query params:

https://docs.djangoproject.com/en/3.0/ref/settings/#std:setting-APPEND_SLASH
https://www.django-rest-framework.org/api-guide/routers/#simplerouter

In both cases jsonapi-client fails to fetch the resource because it either faces the 404 or some headers get lost during the redirection, and response doesn't contain a valid JSON.

Suggestion:

I suggest adding an option to the session that would allow to handle this situation.
I might miss some other places where the trailing slash should be appended though :)

[coveralls]

Coverage decreased (-0.1%) to 88.407% when pulling 0398ee0 on iamanikeev:support-apis-with-trailing-slash-requirement into 9df28f5 on qvantel:master.

[coveralls]

Coverage decreased (-0.1%) to 88.407% when pulling 0398ee0 on iamanikeev:support-apis-with-trailing-slash-requirement into 9df28f5 on qvantel:master.