oops, calendar header tags might be empty

lib/widget/base.rb

@@ -18,6 +18,7 @@ class Widget::Base < Widget
   # Write a string to the canvas. The string is marked as html_safe.
   # This will write twice, if @cache_output is set.
   def write(str)
+    str ||= ""
     @output ||= "".html_safe
     @output.write str.html_safe
     @cache_output.write(str.html_safe) if @cache_output

lib/widget/filters/date.rb

@@ -1,6 +1,6 @@
 class Widget::Filters::Date < Widget::Filters::Base
   def calendar_for(field_id)
-    include_calendar_headers_tags +
+    write include_calendar_headers_tags
     image_tag("calendar.png", {:id => "#{field_id}_trigger",:class => "calendar-trigger"}) +
     javascript_tag("Calendar.setup({inputField : '#{field_id}', ifFormat : '%Y-%m-%d', button : '#{field_id}_trigger' });")
   end