mediawiki: workaround an apache bug

setifempty will work correctly only if we set the 'always'
keyword. Examples in apache's doc suggest otherwise, but I tested this
to remove the duplicate headers

Change-Id: Id0dc821f553bbbca5a8180d59219bfc275eb0b2b
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>