Allow fieldsets to be created without a block.

actionpack/lib/action_view/helpers/form_tag_helper.rb

@@ -525,10 +525,9 @@ def image_submit_tag(source, options = {})
       #   <% end %>
       #   # => <fieldset class="format"><p><input id="name" name="name" type="text" /></p></fieldset>
       def field_set_tag(legend = nil, options = nil, &block)
-        content = capture(&block)
         output = tag(:fieldset, options, true)
         output.safe_concat(content_tag(:legend, legend)) unless legend.blank?
-        output.concat(content)
+        output.concat(capture(&block)) if block_given?
         output.safe_concat("</fieldset>")
       end
 

actionpack/test/template/form_tag_helper_test.rb

@@ -513,6 +513,16 @@ def test_field_set_tag_in_erb
 
     expected = %(<fieldset class="format">Hello world!</fieldset>)
     assert_dom_equal expected, output_buffer
+
+    output_buffer = render_erb("<%= field_set_tag %>")
+
+    expected = %(<fieldset></fieldset>)
+    assert_dom_equal expected, output_buffer
+
+    output_buffer = render_erb("<%= field_set_tag('You legend!') %>")
+
+    expected = %(<fieldset><legend>You legend!</legend></fieldset>)
+    assert_dom_equal expected, output_buffer
   end
 
   def test_text_area_tag_options_symbolize_keys_side_effects