Fix for apps that don't load contenttypes module.

[amw]

PR #319 brought support for generic relations. Unfortunately apps that don't add contenttypes to it's INSTALLED_APPS would crash and burn:

RuntimeError: Model class django.contrib.contenttypes.models.ContentType doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.

I would like to add a test for this, but it would require two separate Django configs. Not sure how you would want to handle that in py.test.

Also using type(something) is object() comparison as a safer alternative to type(something) is type(None). If something happened to be None we would enter a branch that was never supposed to run.

[codecov-io]

Codecov Report

Merging #345 into develop will increase coverage by <.01%.
The diff coverage is 80%.

@@             Coverage Diff             @@
##           develop     #345      +/-   ##
===========================================
+ Coverage    76.37%   76.38%   +<.01%     
===========================================
  Files           50       50              
  Lines         6024     6025       +1     
===========================================
+ Hits          4601     4602       +1     
  Misses        1423     1423

Impacted Files	Coverage Δ
rest_framework_json_api/utils.py	89.79% <80%> (+0.04%)	⬆️

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[mblayman]

I agree that a test would be nice, but this might be one of those cases where the setup for that test would be extreme overkill since it involves things that happen at import time.

What do you think of adding a # pragma: no cover to the if clause that checks for contenttypes in INSTALLED_APPS? It seems like that might be a clue to future readers that this was considered and declared "not worth testing."

[amw]

Added.

[mblayman]

👍