fix: do not call JSON.parse on event.body (#89)

This PR removes the `JSON.parse` call from `event.body`. This was causing issues when trying to invoke Lambda functions asynchronously with `async` set to `true` in a `serverless.yaml` config file. I'm not exactly sure why this doesn't cause issues when `async` is not set to `true`, but I tested these changes with `http` events (both `async: true` and `async: false`) and `httpApi` events and it seemed to work fine on both. Also, I believe the `verifyAndReceive` function internally handles both objects and strings anyway.

Author: ajschmidt8

Diff for: lambda-function.js

@@ -18,7 +18,7 @@ async function lambdaFunction(probot, event, context) {
       signature:
         headersLowerCase["x-hub-signature-256"] ||
         headersLowerCase["x-hub-signature"],
-      payload: JSON.parse(event.body),
+      payload: event.body,
     });
 
     return {