Added entrySeq() to Callbacks map() to fix React 16 + Immutable issue

[geraldglynn]

Description

In order to fix known rendering issue in React 16 with Immutable Maps, added entrySeq() before map()s on Immutable Maps in:
src/core/plugins/oas3/components/callbacks.jsx

Motivation and Context

React 16 renders Immutable Map keys.

How Has This Been Tested?

Tested on use of SwaggerUI within our product which uses React 16.

Built Docker image of our product which uses SwaggerUI package.

Screenshots (if appropriate):

Checklist

My PR contains...

• No code changes (src/ is unmodified: changes to documentation, CI, metadata, etc.)
• Dependency changes (any modification to dependencies in package.json)
• Bug fixes (non-breaking change which fixes an issue)
• Improvements (misc. changes to existing features)
• Features (non-breaking change which adds functionality)

My changes...

• are breaking changes to a public API (config options, System API, major UI change, etc).
• are breaking changes to a private API (Redux, component props, utility functions, etc.).
• are breaking changes to a developer API (npm script behavior changes, new dev system dependencies, etc).
• are not breaking changes.

Documentation

• My changes do not require a change to the project documentation.
• My changes require a change to the project documentation.
• If yes to above: I have updated the documentation accordingly.

Automated tests

• My changes can not or do not need to be tested.
• My changes can and should be tested by unit and/or integration tests.
• If yes to above: I have added tests to cover my changes.
• If yes to above: I have taken care to cover edge cases in my tests.
• All new and existing tests passed.

[char0n]

This conventions gets rid of Immutable structures and assures that Immutables are not React children. More info here: immutable-js/immutable-js#667 (comment)

[char0n]

Thanks!