title: Couldn't find Plan without an ID in subscription controller
body: <p>When I attempt to run the code, I get the error above. 
  This happen on my stripe payment form : 
  Step 1: The user select a plan
  Step 2: The user confirm the current plan
  Step 3: The user pay for this plan 
  The error come in the third step. </p>
  
  <p>Here are the logs :</p>
  
  <pre><code>Started POST "/subscriptions" for 127.0.0.1 at 2015-07-27 08:03:57 +0200
  Processing by SubscriptionsController#new as HTML
    Parameters: {"utf8"=&gt;"✓","authenticity_token"=&gt;"8oQZ2GBhAcAmrDtfbH0SEmbjfzfO2+nfpUtpIySUyTQ=", "subscription"=&gt;{"company"=&gt;"", "adress"=&gt;"", "city"=&gt;"", "country"=&gt;"", "postal_code"=&gt;"", "phone"=&gt;"", "plan_id"=&gt;"52", "stripe_card_token"=&gt;""}, "commit"=&gt;"Terminer la commande"}
  Completed 404 Not Found in 1ms
  
  ActiveRecord::RecordNotFound (Couldn't find Plan without an ID):
    app/controllers/subscriptions_controller.rb:11:in `new'
  </code></pre>
  
  <p>Here is the subscription_controller.rb :</p>
  
  <pre><code>    require "stripe"
  skip_before_filter  :verify_authenticity_token
  skip_before_action :set_plan, only: [:new, :create]
  
  def new
      @titre = "Terminer la commande"
      plan = Plan.find(params[:plan_id])
      @subscription = plan.subscriptions.build(:plan_id =&gt; params[:plan])
  end
  
  def create
      @subscription = Subscription.new(params[:subscription])
      if @subscription.save_with_payment
          flash[:success] = 'Congrats'
          redirect_to nosformations_path
          else
          flash[:error] = "Error"
          redirect_to root_url
      end
  end
  
  private
  
  def set_plan
      @plan = Plan.find(params[:plan_id])
  end
  </code></pre>
  
  <p>Here's the model Subscription.rb :</p>
  
  <pre><code>    belongs_to :plan
  belongs_to :user
  
  validates_presence_of :plan_id
  validates_presence_of :email
  attr_accessible :name, :price, :old_price, :also, :type_of, :interval, :description, :plan_id, :company, :adress, :city, :country, :postal_code, :phone, :stripe_card_token, :coupon
  
  attr_accessor :stripe_card_token
  
  def save_with_payment
      if valid?
          customer = Stripe::Customer.create(description: email, plan: plan_id, card: stripe_card_token, email: email)
          self.stripe_customer_token = customer.id
          save!
      end
      rescue Stripe::InvalidRequestError =&gt; e
      logger.error "Erreur Stripe lors de la création du profil client : #{e.message}"
      errors.add :base, "Le paiement a échoué. Merci de vérifier les paramètres de paiement."
      false
  end
  </code></pre>
  
  <p>I am looking for a response for several hours without success...
  Any idea ? </p>