[core] Handle referenced enum case correctly

[jmini]

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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)

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?

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[wing328]

LGTM

[ajdergute]

Thank's a lot for this superfast bug fix. I tried it today with 4.0.0-beta2 and it works.

[adrien-marsoulaud]

This issue has raised again since 5.1.0 version