load whole config when no environmental keys present in Rails config YML

[progapandist]

What is the purpose of this pull request?

Load configuration in Rails when no "environmental" top keys are present in the config/my_gem.yml

What changes did you make? (overview)

Сheck if the YML contains top-level keys that correspond to Rails environments

Checklist

• [x] I've added tests for this change
• [x] I've added a Changelog entry
• [x] I've updated a documentation

[progapandist]

@palkan — just implemented your suggestions and added more tests. also updated CHANGELOG and README. LMK 🙏

[palkan]

Thanks!