Set refundId

[jcmanzo]

Sets the refundId property when constructing refund from gateway reponse. In other gateways this value would contain the merchant-specified refund id but BlueSnap does not support this. This PR makes sure that getRefundId will return a non-nullable value.

[jcmanzo]

There is a single test that does not pass due to a couple issues I've discovered but feel don't need to be addressed at this point.

• Phpunit fails due to this error https://www.php.net/manual/en/reflectiontype.tostring.php
• The issue shows up because this repo is using an older version of phpunit (4.8.36) due to "needing" to support older php versions.
• The issue is a warning in pre 7.x.x versions but in more recent version starting with 7, this now results in a fatal failure.

We should ideally upgrade this whole library to use the latest php version to avoid these types of problems but that's a larger undertaking and not in scope of what this PR is hoping to achieve.