Alias field_set_tag helper to fieldset_tag

The [field_set_tag][] renders a `<fieldset>` element. At times, the
desire to render a `fieldset` results in calling `fieldset_tag`, only to
be surprised by a `NoMethodError`.

Originally, the method's name was `fieldset_tag` helper (defined in
[0e6c8e5][] in 2007), but was renamed in [73c7083][] (3 days later).

This commit aliases `field_set_tag` to `fieldset_tag` so that both are
available.

Additionally, defines the method so that it utilizes the [content_tag][]
so that it isn't responsible for manually managing the closing
(`</fieldset>`) of the opening `<fieldset>` tag.

[field_set_tag]: https://edgeapi.rubyonrails.org/classes/ActionView/Helpers/FormTagHelper.html#method-i-field_set_tag
[0e6c8e5]: rails@0e6c8e5
[73c7083]: rails@73c7083
[content_tag]: https://edgeapi.rubyonrails.org/classes/ActionView/Helpers/TagHelper.html#method-i-content_tag

Author: seanpdoyle

actionview/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Alias `field_set_tag` helper to `fieldset_tag` to match `<fieldset>` element
+
+    *Sean Doyle*
+
 *   Deprecate passing content to void elements when using `tag.br` type tag builders.
 
     *Hartley McGuire*

actionview/lib/action_view/helpers/form_tag_helper.rb

@@ -675,11 +675,13 @@ 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)
-        output = tag(:fieldset, options, true)
-        output.safe_concat(content_tag("legend", legend)) unless legend.blank?
-        output.concat(capture(&block)) if block_given?
-        output.safe_concat("</fieldset>")
+        content = []
+        content << content_tag("legend", legend) unless legend.blank?
+        content << capture(&block) if block_given?
+
+        content_tag(:fieldset, safe_join(content), options)
       end
+      alias_method :fieldset_tag, :field_set_tag
 
       # Creates a text field of type "color".
       #

actionview/test/template/form_tag_helper_test.rb

@@ -925,6 +925,11 @@ def test_field_set_tag_in_erb
     expected = %(<fieldset class="format">Hello world!</fieldset>)
     assert_dom_equal expected, output_buffer
 
+    output_buffer = render_erb("<%= fieldset_tag('', :class => 'format') do %>Hello world!<% end %>")
+
+    expected = %(<fieldset class="format">Hello world!</fieldset>)
+    assert_dom_equal expected, output_buffer
+
     output_buffer = render_erb("<%= field_set_tag %>")
 
     expected = %(<fieldset></fieldset>)