title: simple_form doesnt prevent sending request without required fields
body: <p>i have this form</p>
  
  <pre><code>= simple_form_for @category.fields.build, url: category_fields_path(@category) do |f|
      = f.input :kind, collection: Field::FIELD_TYPES, prompt: "Choose field type"
      = f.input :description
      = f.submit "Add field"
  </code></pre>
  
  <p>and this field model</p>
  
  <pre><code>class Field &lt; ActiveRecord::Base
    FIELD_TYPES = %w(integer float date string text)
  
    validates :description, presence: true
    validates :kind, presence: true
  
    belongs_to :category
  
  end
  </code></pre>
  
  <p>when i leave 'description' field empty, no request is send and i get notice 'Please fill out this field'. which is what i want. on the other hand, when description is filled in but kind is not, a request is still send to the 'create' action! No field gets created, but 'description' needs to be filled in again. there should be no request in such situation. any idea how to fix this?</p>