it should not unnecessarily stringify string values for text/plain content type

[mathis-m]

Description

Sample for text/plain should not be stringified. Introduced with #6412 the json sample for type string will be stringified. This should only be the case if the content type is not text/plain.

Motivation and Context

Fixes #6430

How Has This Been Tested?

in browser
by new tests

Screenshots (if appropriate):

see #6430

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.

[hkosova]

The logic should probably be changed to apply JSON.stringify specifically to JSON media types (application/json, application/vnd.api+json, etc.) and ignore all other media types.

[mathis-m]

The json sample generation is the default case.
I have refactored the code to clarify the different cases.

[mathis-m]

@hkosova do you think it is better to use the old behavior(only stringify object values) before #6412 was merged as default sample case and introduce case for json content type where sting values will be stringified as well?
text/plain content type sample generation would then implicitly match the default case where only object values will be stringified.

[tim-lai]

@hkosova do you think it is better to use the old behavior(only stringify object values) before #6412 was merged as default sample case and introduce case for json content type where sting values will be stringified as well?
text/plain content type sample generation would then implicitly match the default case where only object values will be stringified.

@mathis-m I this suggestion is better. Revert to original behavior to only stringify object values, but add a function for an allowList for non-object values. e.g if (object || allowList) { stringify }

[mathis-m]

@tim-lai thanks for your input. Will implement this in the next days.

[mathis-m]

@tim-lai could you review the changes made.