Some strings get converted to a number (bug)

Description

Some strings get converted to a number when running swagger-codegen. For example, the following

    phone_number_param_number:
      type: object
      properties:
        number:
          type: string
          required: true
          description: The phone number in E164 format.
          example: !!str "+15558675309"

will become

    phone_number_param_number:
      type: object
      properties:
        number:
          type: string
          required: true
          description: The phone number in E164 format.
          example: +15558675309

which is interpreted by most YAML parsers as the positive number 15558675309.

Swagger-codegen version

3.0.33
3.0.34
3.0.35

Swagger declaration file content or url

    phone_number_param_number:
      type: object
      properties:
        number:
          type: string
          required: true
          description: The phone number in E164 format.
          example: !!str "+15558675309"

Command line used for generation

docker run -v (pwd):/local swagger-codegen-cli-v3 generate -i /local/index.yaml -l openapi-yaml -o /local/combined

Steps to reproduce

Run the above command on a YAML file which includes a string field with value "+15558675309".

Related issues/PRs

Suggest a fix/enhancement

Start with the swagger-codegen-cli-v3 generate entry point and the YAML handling used by the openapi-yaml generator. Reproduce the issue with index.yaml and the supplied command, then verify that the quoted phone-number example remains a string in the generated YAML rather than becoming an unquoted number.