[FEATURE] Enable Content-length header in FE by default

Enabling Content-length header speeds up rendering if pipelining
is enabled in Apache (browser does not have to wait for further
data).

Change-Id: Id6212d0db60732aca035c87e6686711f53eb6fd8
Resolves: #19732
Releases: 6.2
Reviewed-on: https://review.typo3.org/21243
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Thorsten Kahler
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn