Check if escape sequences are handled correctly

[gr2m]

This is a follow up for https://github.com/octokit/webhooks.js/issues/71. It would be good to check if smee is lowercasing escape sequences before forward a webhook event.

[gimenete]

I think it is a bug doing any type of transformation. The payload should not be touched or any signature calculation from github could break as I mentioned here https://github.com/octokit/webhooks.js/issues/71#issuecomment-481430219

[gr2m]

No intended transformation is done, but given that it’s happening with JSON.parse() I want to make sure that smee is not affected