title: stripe credit card form error - the exp_month parameter should be an integer
body: <p>I have a multi-step form with the second page being the billing information. </p>
  
  <p>When I try to press the next button to go to the billing page, I receive the error:</p>
  
  <p>*"the exp_month parameter should be an integer instead is undefined"*</p>
  
  <p>My fields look like this:</p>
  
  <pre><code>&lt;div class="field"&gt;
      &lt;%= label_tag :card_number, "Card Number" %&gt;
      &lt;%= text_field_tag :card_number, nil, name:nil %&gt;
  &lt;/div&gt;
  &lt;br/&gt;
  &lt;div class="field"&gt;
      &lt;%= label_tag :card_code, "Security Code (CVV)" %&gt;
      &lt;%= text_field_tag :card_code, nil, name:nil %&gt;
  &lt;/div&gt;
  &lt;br/&gt;
    &lt;div class="field"&gt;
      &lt;%= label_tag :card_month, "Expiration Date" %&gt;
      &lt;%= select_month nil, {add_month_numbers: true}, {name: nil, id: "card_month"} %&gt;
      &lt;%= select_year nil, {start_year: Date.today.year, end_year: Date.today.year+15}, {name: nil, id: "card_year"} %&gt;
    &lt;/div&gt;
  </code></pre>
  
  <p>CoffeeScript:</p>
  
  <pre><code>jQuery -&gt;
    Stripe.setPublishableKey($('meta[name="stripe-key"]').attr('content'))
    payment.setupForm()
  
  payment =
    setupForm: -&gt;
      $('#new_post').submit -&gt;
        $('input[type=submit]').attr 'disabled', no
        payment.processCard()
        false
  
    processCard: -&gt;
      card =
        number: $('#card_number').val()
        cvc: $('#card_code').val()
        expMonth: $('#card_month').val()
        expYear: $('#card_year').val()
      Stripe.createToken(card, payment.handleStripeResponse)
  
    handleStripeResponse: (status, response) -&gt;
      if status is 200
        alert response.id
      else
        alert response.error.message
  </code></pre>