using Omnipay/paypal/laravel/php trying to generate a card declined using negative testing

Question

Has anyone been able to do a card decline test in paypals sandbox using omnipay and paypal

I have tried a number of different ways. My sandbox account has negative testing enabled and I have tried different amounts that are suppose to trigger failures but nothing seems to create a card decline.

I am setting up OmniPay like::

    $this->gateway = Omnipay::create('PayPal_Rest');
    $this->gateway->setClientId($paypalClientID);
    $this->gateway->setSecret($paypalSecret);
    $this->gateway->setTestMode($testMode); 

One dev page suggested putting something in the http header, but i can not figure out how to add something to the omnipay header

this is the curl example i am trying to reproduce:

 -H 'PayPal-Mock-Response: {"mock_application_codes": "DUPLICATE_INVOICE_ID"}'

how do i add header values to the omnipay request??

Thanks