Add a patch for Django bug 19611.

https://code.djangoproject.com/ticket/19611
Don't fail to parse text/html which declares a charset, like the admin interface.
Add a check for empty responses, before trying to parse them.
Print markers around the invalid content to help identify it visually in test output.
Add a helper to convert an admin form into a normal form.

So that you can pass it to update_form_values().
Add support to update_form_values for ReadOnlyPasswordHashWidget.

As used by admin user editing form.
Fix handling of drop-down lists with no options in them.
Improve assertion for admin form with errors.

Allow ignoring errors or asserting that there are none.