[core] fix referenced enum case

[jmini]

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Description of the PR

Fixes #2127

Follow up of PR #1981

When you have a default value defined, in the outer enum case (where a seperated enum file is created) the default value was only working for the string type.

This PR address the issue.

---

Tested with a spec having this outer enum cases:

components:
  schemas:
    IntEnum:
      type: integer
      format: int32
      default: 2
      enum:
        - 1
        - 2
        - 3

    LongEnum:
      type: integer
      format: int64
      default: 30
      enum:
        - 20
        - 30
        - 40

    StringEnum:
      type: string
      default: "b"
      enum:
        - "c"
        - "b"
        - "a"

    EnumWithCustomName:
      type: integer
      format: int32
      default: 2
      enum:
        - 1
        - 2
      x-enum-varnames:
        - FOO
        - BAR

And following schema:

    ObjWithEnumProperties:
      type: object
      properties:
        iprop:
            $ref: "#/components/schemas/IntEnum"
        lprop:
            $ref: "#/components/schemas/LongEnum"
        sprop:
            $ref: "#/components/schemas/StringEnum"
        custom:
            $ref: "#/components/schemas/EnumWithCustomName"

The change is made in the core and I think that a lot of generators are impacted.
I have tested it for different java generators.
Setting a modelNamePrefix should also be considered during tests.

I recommend to try other generators/config as well.

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[fuj1g0n]

great work!
I installed your branch locally and confirmed it works perfectly :)

[wing328]

@jmini thanks for the PR 👍

@fujigon thanks for testing the fix.