[BUGFIX] Ensure PageContentErrorHandler content is served correctly

Use only the body and the content-type header of the internal
sub-request. Everything else of the sub-request can be discarded.

Resolves: #91582
Related: #81644
Releases: master, 10.4
Change-Id: I393acb340c04b3c7aa0cde1e1de1df9fcc92ca7a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64672


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Johannes Kasberger <johannes.kasberger@reelworx.at>
Reviewed-by: Benni Mack <benni@typo3.org>