Rails MailForm Gem captcha validation not passing test

I am using the MailForm Gem to create a contact form for my app and everything seems to be working just fine so I decided to write some tests to make sure it stays that way.

class ContactsControllerTest < ActionDispatch::IntegrationTest
  def setup
    ActionMailer::Base.deliveries.clear
  end

  test "should send contact email" do
    get contact_path
    post contacts_path, params: { contact: {
                name: "interested customer",
                email: "interested@customer.com",
                subject: "we are interested!",
                message: "we are so interested!!!" 
    }}  
    assert_equal 1, ActionMailer::Base.deliveries.size
    assert_redirected_to root_path
  end

  test "should not send invalid contact email" do
    get contact_path
    post contacts_path, params: { contact: {
                name: "",
                email: "",
                subject: "",
                message: "" 
    }}  
    assert_equal 0, ActionMailer::Base.deliveries.size
    assert_template 'contacts/new'

  end

  test "should not send contact email with captcha filled" do
    get contact_path
    post contacts_path, params: { contact: {
                name: "interested customer",
                email: "interested@customer.com",
                subject: "we are interested!",
                message: "we are so interested!!!",
                nickname: "not_blank" 
    }}  
    assert_equal 0, ActionMailer::Base.deliveries.size
    assert_template 'contacts/new'
  end

the first two tests pass while the third fails with the message

FAIL["test_should_not_send_contact_email_with_captcha_filled", ContactsControllerTest, 5.2574800989968935]
    test_should_not_send_contact_email_with_captcha_filled#ContactsControllerTest (5.26s)
    Expected: 0
      Actual: 1
    test/controllers/contacts_controller_test.rb:35:in `block in <class:ContactsControllerTest>'

My model looks like this.

class Contact < MailForm::Base
   attribute :name,      :validate => true
   attribute :email,     :validate => /\A([\w\.%\+\-]+)@([\w\-]+\.)+  ([\w]{2,})\z/i
   attribute :subject
   attribute :message
   attribute :nickname, :captcha => true

   def headers
    {
      :subject => "Contact: #{subject}" ,
      :to => "myemail@mail.com",
      :from => %("#{name}" <#{email}>)
     }
    end
end

My first thought was that the captcha validation is not stopping the mail from being sent. If someone could point out what I am missing I would appreciate it.

The mailform model returns valid, even if the nickname is given. But you can check if it is spam, which prohibits the mail from being sent. I use this to check if the spam detection works(with rspec):

it 'should be marked spam if it contains :nickname' do
 expect(FactoryGirl.build(:contact_with_nickname)).to be_spam
end