[BUG] String interpolation in Ruby client code dose not work

[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 interpolation in the error message of Ruby's client dose not work.

Swagger-codegen version

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

$ git log --oneline -n 1
0b66fa5c82 (HEAD -> master, origin/master, origin/HEAD) add servers to path, operation (#2048)

Swagger declaration file content or url

https://gist.github.com/autopp/5a7375cd9b521b5e5580ba3b1df40c75

The point to notice in this example contains enum in the model.

Command line used for generation

$ java -jar modules/swagger-codegen-cli/target/swagger-codegen-cli.jar generate -i 'https://gist.githubusercontent.com/autopp/5a7375cd9b521b5e5580ba3b1df40c75/raw/ca1d5eccce457c7337922b4a6a74f002848e4b4c/swagger.yml' -l ruby -o /tmp/gem

Steps to reproduce

• Run the above command to generate client code for Ruby.
• Generated /tmp/gem/lib/openapi_client/models/x.rb contains string interpolation notation in single-quote string. (It dose not work!)

  def y=(y)
    validator = EnumAttributeValidator.new('String', ['a'])
    unless validator.valid?(y)
      fail ArgumentError, 'invalid value for "y", must be one of #{validator.allowable_values}.' # <= HERE!
    end
    @y = y
  end

Related issues/PRs

Suggest a fix/enhancement

The corresponding template is this.

I think that it should be a double-quote string.

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