Making the body of a function use a mock of an object

I have the following method in a controller:

def webhook
    data_json = JSON.parse(request.body.read) # it comes from the test, it's OK
    event = Stripe::Event.retrieve(data_json[:id]) # it's not OK, it's a real request to Stripe
    stripe_cust_id = event.data.object.customer
    user = User.where(stripe_customer_id: stripe_cust_id)
    #.....

In a spec file I create a mock for event and then make a post request to webhook in a test. I'm not allowed to change the body or signature of webhook because I'm testing it. So how do I make it use the mock I create?

describe '#webhook' do
    it 'something' do
      user = FactoryGirl.create(:user)
      event = StripeMock.mock_webhook_event('invoice.payment_succeeded')
      post(:webhook, event.to_json)

      #error because webhook makes a real request to Stripe