title: Devise find_for_authentication not firing with multiple authentication keys
body: <p>I have two devise models: User and Member</p>
  
  <p>As such, I'm specifying authentication keys on the models themselves instead of in the Devise initializer.</p>
  
  <p>Member.rb </p>
  
  <pre><code>devise :database_authenticatable, :registerable,
      :recoverable, :rememberable, :trackable, :validatable, :omniauthable, :authentication_keys =&gt; [:email, :subdomain]
  </code></pre>
  
  <p>I also override </p>
  
  <pre><code>  def self.find_for_authentication(conditions={})
      debugger
      conditions[:account_id] = Account.find_by_subdomain(conditions.delete(:subdomain)).id
      super(conditions)
    end
  </code></pre>
  
  <p>Unfortunately, when authentication_keys has multiple keys, my find_for_authentication method doesn't appear to be firing.  Works fine when I specify just one key.  Any thoughts?</p>