[BUG][R]Some Model classes are not generating in R-generator like "Date"

[Ramanth]

Bug Report Checklist

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• Have you validated the input using an OpenAPI validator (example)?
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Description

Model is not getting generated when the name of the model is "Date" in r codegeneration.

Details of what causing the issue:
Date is being mapped to "java.util.Date" in DefaultCodegen file which is being extended in the RClientCodegen as a result the Date is mapped to the Date class of java and is being skipped from being generated.

I guess the importMapping list needs to be cleared in the RClientCodegen constructor as it makes no sense of converting them to java types.

openapi-generator version

4.0.0

OpenAPI declaration file content or url

Command line used for generation

Steps to reproduce

use openapi-codegen-cli to generate the models in r language, with any model name matching "Date"
The model will not be generated.

Related issues/PRs

Not Yet

Suggest a fix

Clear the importMappping list inside the constructor of RClientCodegen

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[wing328]

Already fixed via https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/RClientCodegen.java#L179

[wing328]

FYI. Added a test via #13345