Fix validateWebhook backwards incompatibility with older Node.js (#298)

* Use backward compatible syntax for accessing object keys on requestData.headers

* Stringifying requestData.body when it is of type object. This is  backward compatibility for Next.js 12.1.x / Node.js v18

Author: jakedahn

lib/util.js

@@ -35,9 +35,15 @@ async function validateWebhook(requestData, secret) {
   const signingSecret = secret || requestData.secret;
 
   if (requestData && requestData.headers && requestData.body) {
-    id = requestData.headers.get("webhook-id");
-    timestamp = requestData.headers.get("webhook-timestamp");
-    signature = requestData.headers.get("webhook-signature");
+    id =
+      requestData.headers["webhook-id"] ||
+      requestData.headers.get?.("webhook-id");
+    timestamp =
+      requestData.headers["webhook-timestamp"] ||
+      requestData.headers.get?.("webhook-timestamp");
+    signature =
+      requestData.headers["webhook-signature"] ||
+      requestData.headers.get?.("webhook-signature");
     body = requestData.body;
   }
 
@@ -49,6 +55,8 @@ async function validateWebhook(requestData, secret) {
     }
   } else if (isTypedArray(body)) {
     body = await new Blob([body]).text();
+  } else if (typeof body === "object") {
+    body = JSON.stringify(body);
   } else if (typeof body !== "string") {
     throw new Error("Invalid body type");
   }