Default value ignored in user profile field (Text input) on signup page

When you create a text input user profile field and expose it to the user signup form the default value configured is not rendered.

Steps to reproduce

1. Create a 'Text input' user profile field. (/user/profile/index.php)
2. Set 'Display on signup page?' to 'Yes'.
3. Give the 'Default value' element some text.
4. Hit 'Save changes'
5. Logout and navigate to the user signup page (/login/signup.php)

Expected result
The created text input element to contain the configured default value.

Actual result
The created text input element is empty.