[Ruby] Add model name mapping support to ruby client generator

[tschoppi]

The modelNameMappings were introduced with PRs #16209, #16234 and are now also used in the ruby client generator.

The previously implemented lookup in schemaMappings (PR #12962) still takes precedence.

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[tschoppi]

@cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[tschoppi]

Thanks for the hint, @wing328 . I have changed the author information in the commit accordingly.

[wing328]

LGTM. thanks for the enhancement.