[ci skip] Fix which method `FormHelper#date_field` try to call

When this method was implemented
(https://github.com/rails/rails/pull/5016/files), `to_date` is called.
But this behavior was chagned refactoring
(https://github.com/rails/rails/pull/6452/files).
In the first commit, there were not test which asserts `to_date` is called.

I think trying `to_date` is more useful than trying `strftime`, because
we can write `"2015-01-01".to_date`. But first fix comments to match
actual behavior.

actionview/lib/action_view/helpers/form_helper.rb

@@ -1038,7 +1038,7 @@ def telephone_field(object_name, method, options = {})
       #   date_field("user", "born_on")
       #   # => <input id="user_born_on" name="user[born_on]" type="date" />
       #
-      # The default value is generated by trying to call "to_date"
+      # The default value is generated by trying to call +strftime+ with "%Y-%m-%d"
       # on the object's value, which makes it behave as expected for instances
       # of DateTime and ActiveSupport::TimeWithZone. You can still override that
       # by passing the "value" option explicitly, e.g.