Generate Class#openapi_nullable methods

[biglovisa]

What

Fixes #5. Adding generated method .openapi_nullable to models

Why

This method was removed ~9 months back and was breaking the #to_hash method on the models.

SDK Release Checklist

• Have you added an integration test for the changes?
• Have you built the gem locally and made queries against it successfully?
• Did you update the changelog?
• Did you bump the package version?
• For breaking changes, did you plan for the release of the new SDK versions and deploy the API to production?

[bspellacy]

This looks good - can we add a test for the methods that weren't working before to prevent this from happening again?

[bspellacy]

Sorry - I should have been more clear! The specs you added look great, but it was my understanding that #to_hash and #to_h were broken and those are the methods we should test.

[biglovisa]

Ah yes - makes more sense, thanks for clarifying! I took the opportunity to create some shared examples instead for the generated classes. Currently we only cover the .openapi_nullable and #to_hash methods, but this can be added on to over time.