[BUGFIX] @charset must be lowercase in CSS

CSS files must comply to http://www.w3.org/TR/CSS21/syndata.html#charset
which requires @charset to be
"the 10 characters '@charset "' (lowercase, no backslash escapes)".
Also the CSS compressor looks for the lowercase version.

Change-Id: I71436dfd4a74da8ee5592e6755565d96d160d1fe
Fixes: #32163
Releases: 4.7, 4.6, 4.5, 4.4
Reviewed-on: http://review.typo3.org/6979
Reviewed-by: Jan
Tested-by: Jan
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Simon Schaufelberger
Tested-by: Simon Schaufelberger
Reviewed-by: Oliver Klee
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer