Add test for explicitly setting required option to false in apply_schema
user
```ruby
require 'test_helper'

class DataMapperSchemaTest < ActiveSupport::TestCase
  test 'required explicitly set to false' do
    DataMapper::Property.required(true)
    model = Class.new(User)
    model.apply_devise_schema :required_string, String, :required => true
    model.apply_devise_schema :not_required_string, String
    assert model.properties['required_string'].required?
    assert !model.properties['not_required_string'].required?
    DataMapper::Property.required(false)
  end
end
```
assistant