Create subscription with coupon

[mhlsf]

Hello,

We can't create a subscription with a coupon in the parameters. Settings the coupon as a parameter in the request allow you to set a discount on a newly created subscription as it is stated in the documentation.

We should update the CreateSubscriptionRequest and add getter and setter for the coupon as follow :

`/**
* Set the coupon
*
* @param $value
* @return \Omnipay\Common\Message\AbstractRequest|CreateSubscriptionRequest
*/
public function setCoupon($value)
{
return $this->setParameter('coupon', $value);
}

/**
 * Set the plan
 *
 * @param $value
 * @return \Omnipay\Common\Message\AbstractRequest|CreateSubscriptionRequest
 */
public function setPlan($value)
{
    return $this->setParameter('plan', $value);
}

`
And also add a condition the getData() method to add the coupon in the request data like this :

if($this->parameters->has('coupon')) { $data['coupon'] = $this->getCoupon(); }

I will try to make a proper pull request

Thank you