add test-case for link_to_if behavior with a block.

This illustrates the purpose of the block for `link_to_if` and
`link_to_unless` helper methods.

It should help to prevent further mistakes like #19844.

actionview/test/template/url_helper_test.rb

@@ -379,6 +379,11 @@ def test_link_to_if
     assert_dom_equal %{<a href="/">Listing</a>}, link_to_if(true, "Listing", url_hash)
   end
 
+  def test_link_to_if_with_block
+    assert_equal "Fallback", link_to_if(false, "Showing", url_hash) { "Fallback" }
+    assert_dom_equal %{<a href="/">Listing</a>}, link_to_if(true, "Listing", url_hash) { "Fallback" }
+  end
+
   def request_for_url(url, opts = {})
     env = Rack::MockRequest.env_for("http://www.example.com#{url}", opts)
     ActionDispatch::Request.new(env)
@@ -479,6 +484,11 @@ def test_link_unless_current
       link_to_unless_current("Listing", "http://www.example.com/")
   end
 
+  def test_link_to_unless_with_block
+    assert_equal %{<a href="/">Showing</a>}, link_to_unless(false, "Showing", url_hash) { "Fallback" }
+    assert_dom_equal "Fallback", link_to_unless(true, "Listing", url_hash) { "Fallback" }
+  end
+
   def test_mail_to
     assert_dom_equal %{<a href="mailto:david@loudthinking.com">david@loudthinking.com</a>}, mail_to("david@loudthinking.com")
     assert_dom_equal %{<a href="mailto:david@loudthinking.com">David Heinemeier Hansson</a>}, mail_to("david@loudthinking.com", "David Heinemeier Hansson")