fix(readRawBody): resolve cached promise before normalizing buffer

closes #369

Author: pi0

src/utils/body.ts

@@ -30,9 +30,8 @@ export function readRawBody<E extends Encoding = "utf8">(
     (event.node.req as any)[RawBodySymbol] ||
     (event.node.req as any).body; /* unjs/unenv #8 */
   if (_rawBody) {
-    const promise = Promise.resolve(
-      Buffer.isBuffer(_rawBody) ? _rawBody : Buffer.from(_rawBody)
-    );
+    const promise = Promise.resolve(_rawBody)
+      .then(_resolved => Buffer.isBuffer(_resolved) ? _resolved : Buffer.from(_resolved))
     return encoding
       ? promise.then((buff) => buff.toString(encoding))
       : (promise as Promise<any>);