[BUG] [Ruby] Ruby's type mapping about of boolean is not convenient

Created by: autopp

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

The type mappings of Ruby client generator maps boolean to BOOLEAN instead of Boolean. This is not suitable for YARD tag. https://github.com/OpenAPITools/openapi-generator/blob/master/samples/openapi3/client/petstore/ruby/lib/petstore/api/fake_api.rb#L70 See: https://github.com/OpenAPITools/openapi-generator/pull/2329#discussion_r264486738

openapi-generator version

$ java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar version
4.0.0-SNAPSHOT

$ git log --oneline -n 1
302b2fafff (HEAD -> fix-type-mapping-for-ruby, origin/master, origin/HEAD, master) [haskell-http-client] fix bug in test for Maybe A.Value from aa7ea8bdca (#2382)

OpenAPI declaration file content or url

N/A

Command line used for generation

N/A

Steps to reproduce

See: https://github.com/OpenAPITools/openapi-generator/blob/master/samples/openapi3/client/petstore/ruby/lib/petstore/api/fake_api.rb#L70

Related issues/PRs

#2329

Suggest a fix

See: https://github.com/OpenAPITools/openapi-generator/pull/2329#discussion_r264745914