Remove setting default_mimetype in Slim 4

[akrabat]

I think we should not set the default_mimetype in finalize in Slim 4 and document that it should be cleared.

Alternatively, we should only set it to black if it's an empty response.

See #2225.

[akrabat]

We could also set it in the Skeleton, so that it's easy to remove?

[odan]

Maybe add a (optional) middleware for that?

[akrabat]

Replaced with #2396

[Fritofour]

Hi,
Is there a release planified for this issues?
Thanks a lot!