[BUGFIX] Uppercase CSS tag selectors not supported in some mime types

css_styled_content is using uppercase CSS tag selectors. These are not
supported using XHTML with the MIME-Type application/xhtml+xml. The XML
parser simply ignores these selectors.

This patch fixes this issue by changing all uppercase CSS selectors in
css_styled_content to lowercase.

Change-Id: I43ea53a1893d03dac6686f519faebc01c2e8f971
Fixes: #40501
Releases: 4.7, 6.0, 6.1, 6.2
Reviewed-on: https://review.typo3.org/20723
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind