[BUG][JavaScript] client generator: Reserved words detector is not case-sensitive

Created by: delenius

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?

Description

The JS client generator is case-insensitive when it comes to detecting reserved words, while JavaScript itself is not. For example, it complains about 'InstanceOf' as a model name, and renames it to 'ModelInstanceOf'.

openapi-generator version

5.1.0-SNAPSHOT, commit 976b4aa757e989fbaa28f92a721ade39735332e6.

OpenAPI declaration file content or url

See full details in this test repo.

Suggest a fix

The culprit is this line in DefaultCodegen.java:

    protected boolean isReservedWord(String word) {
        return word != null && reservedWords.contains(word.toLowerCase(Locale.ROOT));
    }

Removing .toLowerCase here should fix the problem. I will submit a PR.