Fixed type assignment for referenced enums

Created by: DonDi94

Change-Id: Ic6e87bfdfa58a4a8f6f1f3aac043c57260ed873c

PR checklist

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• 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 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\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
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Description of the PR

Referencing to issue #2923

The change is in DefaultCodegen, so I haven't ran any language-specific script.

The referenced enum schema was missing both the type checking and the assignment of the variable isEnum. I moved the typechecking of the property to an external function which I then use for both the properties p and referencedSchema.

I have been using this fix for a personal project and it has been working well so far. I also tested the case in the issue request and it is solved. The library builds correctly with mvn clean install, but sadly I couldn't test through mvn verify -Psamples as it stops on language specific tests because I am missing the needed tools and running it in a protected environment.