Minimal APIs - return string isn't setting content type header

[LadyNaggaga]

To Reproduce

• Create a new empty web application
• Add swagger UI
• Run the application go to https://localhost:5001/swagger/index.html

[BrennanConroy]

Triage: We might consider setting the response type to "text/plain" by default if it isn't set already.

Thanks for contacting us.

We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[halter73]

Only do this if the Content-Type response header is not already set.