Added a check to the menu's current-Twigfilter.

When you set up your page entrance url at the general site config (param
"homepage"), the current-filter won't catch this particular case and you
wont be able to mark your welcome page as active in the menu. Pretty
simple fix.
Merge pull request #968 from doertedev/master

Added a check to the menu's current-Twigfilter.