[BUGFIX] Correct invalid last-modified header in HTTP response

Missing space between timestamp and timezone abbreviation added when
EXT:form sends date/time value in HTTP header "Last-Modified:" in
response to AJAX calls (saveAction and loadAction).

This patch corrects the invalid date/time value and makes the HTTP header
RFC2616-conform.

Change-Id: I54974266ca470fb7e579bc33f5ca29fa6c5efee2
Resolves: #71749
Releases: master
Reviewed-on: https://review.typo3.org/44852


Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>