Yield translation to `FormBuilder#button` block

When translating a `<button>` element's contents, it is tedious  to make
the translation text available to a block scope.

For instance, when rendering a `<button type="submit">` with an SVG
element as its child, passing translated label text to that SVG
element's [`<title>`][svg-title] element requires an extra call to
`I18n.translate`.

Prior to this commit, doing so would require a double lookup of the
translation key:

```erb
<%# one time here, implicitly %>
<%= form.button do %>
  <svg>
    <title>
      <!-- one time here, explicitly -->
      <%= translate("helpers.submit.post.create") %>
    </title>
    <!-- ... -->
  </svg>
<% end %>
```

This commit modifies the `ActionView::Helpers::FormBuilder#button` to
check for invocations that are passed a block, and conditionally yield
the contents of `submit_default_value` as the argument.

The new view code might look something like this:

```erb
<%= form.button do |text| %>
  <svg>
    <title><%= text %></title>
    <!-- ... -->
  </svg>
<% end %>
```

Callers of the helper are still free to omit the block parameter.

[svg-title]: https://developer.mozilla.org/en-US/docs/Web/SVG/Element/title

actionview/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Yield translated strings to calls of `ActionView::FormBuilder#button`
+    when a block is given.
+
+    *Sean Doyle*
+
 *   Alias `ActionView::Helpers::Tags::Label::LabelBuilder#translation` to
     `#to_s` so that `form.label` calls can yield that value to their blocks.
 

actionview/lib/action_view/helpers/form_helper.rb

@@ -2496,10 +2496,22 @@ module ActionView
       #   #      <strong>Ask me!</strong>
       #   #    </button>
       #
+      #   button do |text|
+      #     content_tag(:strong, text)
+      #   end
+      #   # => <button name='button' type='submit'>
+      #   #      <strong>Create post</strong>
+      #   #    </button>
+      #
       def button(value = nil, options = {}, &block)
         value, options = nil, value if value.is_a?(Hash)
         value ||= submit_default_value
-        @template.button_tag(value, options, &block)
+
+        if block_given?
+          value = @template.capture { yield(value) }
+        end
+
+        @template.button_tag(value, options)
       end
 
       def emitted_hidden_id? # :nodoc:

actionview/test/template/form_helper/form_with_test.rb

@@ -365,6 +365,18 @@ class FormWithActsLikeFormForTest < FormWithTest
     assert_dom_equal expected, output_buffer
   end
 
+  def test_form_with_button_yields_translation
+    form_with(model: @post) do |f|
+      concat(f.button { |value| concat content_tag(:span, value) })
+    end
+
+    expected = whole_form("/posts/123", method: :patch) do
+      "<button name='button' type='submit'><span>Update Post</span></button>"
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_form_with_not_outputting_ids
     old_value = ActionView::Helpers::FormHelper.form_with_generates_ids
     ActionView::Helpers::FormHelper.form_with_generates_ids = false
@@ -981,9 +993,7 @@ class FormWithActsLikeFormForTest < FormWithTest
     form_with(model: Post.new) do |f|
       concat(
         f.label(:title) do |builder|
-          concat tag.span(builder, {
-            class: ("new_record" unless builder.object.persisted?)
-          })
+          concat tag.span(builder, class: ("new_record" unless builder.object.persisted?))
         end
       )
     end