Revert "[BUGFIX] PSR-7: Only send headers if no other headers are prepared"

The AJAX functionality in the backend does not work anymore, thus reverting to the original state, finding a proper solution to solve the problem.

This reverts commit 8e9a766d.

Change-Id: Ie0b563c5973a0ea4142c310fbba304c53a114840
Reviewed-on: http://review.typo3.org/42059


Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>