[Ruby] Escape string interpolation notation of Ruby (#2261)

Created by: autopp

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh, ./bin/security/{LANG}-petstore.sh and ./bin/openapi3/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language. @cliffano (2017/07) @zlx (2017/09) @autopp (2019/02)

Description of the PR

Fix #2261 (closed). This PR has been modified to escape string interpolations in enum when generating Ruby client.

• To escape #{, modified AbstractRubyCodegen::escapeUnsafeCharactors
• Added test class for AbstractRubyCodegen
  • For this the visibility of AbstractRubyCodegen and its constructor ware changed to public.

E.g.

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate --skip-validate-spec -g ruby -o /tmp/gem -i 'https://gist.githubusercontent.com/autopp/48160b45f1c17c950ba72feb971786c3/raw/974579f49d9a0eebff0455e54efce842eee64c9b/example.yml'

lib/openapi_client/api/default_api.rb generated by the above command no longer includes string interpolation.

    def foo_get_with_http_info(opts = {})
      if @api_client.config.debugging
        @api_client.config.logger.debug 'Calling API: DefaultApi.foo_get ...'
      end
      allowable_values = ["^injected code: \#{injected.code}$"]
                                           ^^^ Escaped

NOTE: Unfortunately #2088 (closed) has not been fixed yet.