Add test cases for scenarios related to one-to-one relationships

[anp-scp]

To make this project stable and production ready we need to test it a lot. We are looking to expand the test suite. Writing a test case would be quite simple.

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The test cases in this category includes all such scenarios related to the behaviour mentioned in documentation related to one-to-one relationships.

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Check existing examples for how to write the test cases. They are in: https://github.com/anp-scp/django-to-rest/tree/master/tests

Check below links for more help:

1. Introducing automated testing
2. Testing - Django REST Framework

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