title: Django crispy-forms field_class not working as expected
body: <p>I'm building simple from with <code>crispy-forms</code> but <code>field-class</code> attribute doesn't work as expected.</p>
  
  <p><strong>Form</strong></p>
  
  <pre><code>class ArrivalForm(forms.Form):
      def __init__(self, *args, **kwargs):
          super(ArrivalForm, self).__init__(*args, **kwargs)
          self.helper = FormHelper()
          self.helper.form_id = 'arrival-form'
          self.helper.form_class = ''
          self.helper.form_method = 'POST'
          self.helper.form_action = ''
          self.helper.field_class = 'form-control'
  
          self.helper.layout = Layout(
          Div(
              Div('passenger_name', css_class='col-md-6'),
              Div('passenger_lastname', css_class='col-md-6'),  
              css_class='row'), 
          )
  
          self.helper.add_input(Submit('submit', 'Submit'))
  
      passenger_name = forms.CharField(
          label = "Firstname:",
          max_length = 80,
          required = True,
      )
  
      passenger_lastname = forms.CharField(
          label = "Lastname:",
          max_length = 80,
          required = True,
      )
  </code></pre>
  
  <p><strong>Rendered HTML</strong></p>
  
  <pre><code>&lt;form id="arrival-form" method="post" name="arrival-form"&gt;
      &lt;input name="csrfmiddlewaretoken" type="hidden" value=
      "akxd0BVQrwaHbHr4FLjaDLz72BUUN9rQ"&gt;
      &lt;div class="row"&gt;
          &lt;div class="col-md-6"&gt;
              &lt;div class="control-group" id="div_id_passenger_name"&gt;
                  &lt;label class="control-label requiredField" for=
                  "id_passenger_name"&gt;Firstname:&lt;span class=
                  "asteriskField"&gt;*&lt;/span&gt;&lt;/label&gt;
                  &lt;div class="controls"&gt;
                      &lt;input class="textinput textInput" id="id_passenger_name"
                      maxlength="80" name="passenger_name" type="text"&gt;
                  &lt;/div&gt;
              &lt;/div&gt;
          &lt;/div&gt;
          &lt;div class="col-md-6"&gt;
              &lt;div class="control-group" id="div_id_passenger_lastname"&gt;
                  &lt;label class="control-label requiredField" for=
                  "id_passenger_lastname"&gt;Lastname:&lt;span class=
                  "asteriskField"&gt;*&lt;/span&gt;&lt;/label&gt;
                  &lt;div class="controls"&gt;
                      &lt;input class="textinput textInput" id=
                      "id_passenger_lastname" maxlength="80" name=
                      "passenger_lastname" type="text"&gt;
                  &lt;/div&gt;
              &lt;/div&gt;
          &lt;/div&gt;
      &lt;/div&gt;
      &lt;div class="form-actions"&gt;
          &lt;input class="btn btn-primary" id="submit-id-submit" name="submit"
          type="submit" value="Submit"&gt;
      &lt;/div&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p><strong>Problem</strong>
  I'm expecting <code>input</code> fields to have <code>form-control</code> class added automatically.</p>