Use attr_accessible if ProtectedAttributes gem is present

This fixes the  Rails 4 compatibility work for those who have "protected_attributes" gem installed.

We ran into this issue because a gem had a dependency on the `protected_attributes` gem for Rails 4 compatibility and it was listed in our Gemfile. It was causing odd issues described in issue #287.

This corrects the problem so now any user who happens to have the `protected_attributes` gem anywhere in their dependency graph can still use doorkeeper side-by-side without issues.