fix ActionView::Template::Error (undefined method `title' for 1:Fixnum)

With https://github.com/pharmazone/redmine_charts2/tree/redmine21 we have a page where @page is a Fixnum => Only execute when @page is a wiki page.
Merge pull request #5 from jlgeering/master

fix ActionView::Template::Error (undefined method `title' for 1:Fixnum)