[TYPO3-core] RFC: #11397: [Feature] Add class for render HTML header/footer for BE and FE

[Steffen Kamper]

Hi Steffen(s),

thx for testing!
Indeed the xmlns has a missing space which i fixed here.
The warning with additionalFooterdata takes me wonder, it's a copy of 
additionalHeaderData, but it seems that this is filled all the time. So 
i added an is_array-check to be safe.

I'll send an updated patch if no more problems occur.

vg Steffen