[BUGFIX] Some unit tests fail with "Cannot modify header information"

Method template::endPage() should only send Content-Encoding header
if no headers have been sent to prevent fatal errors.

Change-Id: I57b84ac777a3e5dd7be601a670da7601116fd035
Fixes: #28083
Reviewed-on: http://review.typo3.org/3237
Reviewed-by: Steffen Gebert
Tested-by: Steffen Gebert
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers