🚑️ Inherit content-type from options first

[AlexV525]

Fix the regression of content-type that is brought from #46.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures

Additional context and info (if any)

The proper order to determine which content-type should be used is:

1. Options.headers['content-type']
2. Options.contentType
3. BaseOptions.contentType
4. Headers.jsonContentType

And after #46, it's been changed to:

1. Options.headers['content-type']
2. Options.contentType
3. Headers.jsonContentType

Which dropped the content type set in the BaseOptions.