Fix exception when a server has no roles

Author: tomhughes

_plugins/server_description_filter.rb

@@ -2,7 +2,7 @@ module Jekyll
   module ServerDescriptionFilter
     def server_description(roles)
       descriptions = @context.registers[:site].data['descriptions']
-      roles.map { |role| descriptions[role] }.compact.join('<br>')
+      Array(roles).map { |role| descriptions[role] }.compact.join('<br>')
     end
   end
 end