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



### To Reproduce


- Create a new empty web application 
-  Add swagger UI 
- Run the application  go to  `https://localhost:5001/swagger/index.html`


![image](https://user-images.githubusercontent.com/2546640/123486901-f2525180-d5da-11eb-844b-edba0e015884.png)
