#6545: Fixing several bugs for Java model generation for enums

[sdoeringNew]

PR checklist

• [x] Read the contribution guidelines.
• [x] 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).
• [x] Filed the PR against the correct branch: 2.3.0 branch for changes related to OpenAPI spec v?. Default: master ~Filed the PR against the correct branch: 3.0.0 branch for changes related to OpenAPI spec 3.0. Default: master.~
• [x] Copied the technical committee to review the pull request if your PR is targeting a particular programming langauge: @bbdouglas @JFCote @sreeshas @jfiala @lukoyanov

Description of the PR

fixes #6545

[sdoeringNew]

Something is wrong with the Travis build. It's always timeouting.