[core] Handle referenced enum case correctly

Created by: jmini

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 and ./bin/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. @OpenAPITools/generator-core-team

Description of the PR

This PR is about handling outer-enum with default value correctly.

allowableValues needs to be set, even for referenced enum.

---

Test spec (proposed in #1981 (closed))

openapi: 3.0.1
info:
  title: issue1981 test
  version: '1.0'
servers:
  - url: 'http://localhost:8000/'
paths:
  /testenum:
    post:
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/TestObject'
      responses:
        200:
          description: OK
components:
  schemas:
    TestObject:
      type: object
      properties:
        myEnum:
          $ref: '#/components/schemas/MyEnum'
    MyEnum:
      type: string
      default: FIRSTVALUE
      enum:
      - FIRSTVALUE
      - SECONDVALUE

Diff:

--- a/issue1981-oapigen-spring-boot-spring/src/main/java/org/openapitools/model/TestObject.java
+++ b/issue1981-oapigen-spring-boot-spring/src/main/java/org/openapitools/model/TestObject.java
 
 public class TestObject   {
   @JsonProperty("myEnum")
-  private MyEnum myEnum = null;
+  private MyEnum myEnum = MyEnum.FIRSTVALUE;
 
   public TestObject myEnum(MyEnum myEnum) {
     this.myEnum = myEnum;

@ajdergute have you a way to test/review this PR?