fix: signingKey should not be part of the computed hash

In some flows it `signingKey` is serialized to `null` and
messes up the signature.

Author: mkleene

lib/tdf3/src/assertions.ts

@@ -43,7 +43,7 @@ export type AssertionPayload = {
  * @returns the hexadecimal string representation of the hash
  */
 export async function hash(a: Assertion): Promise<string> {
-  const result = canonicalizeEx(a, { exclude: ['binding', 'hash', 'sign', 'verify'] });
+  const result = canonicalizeEx(a, { exclude: ['binding', 'hash', 'sign', 'verify', 'signingKey'] });
 
   const hash = await crypto.subtle.digest('SHA-256', new TextEncoder().encode(result));
   return hex.encodeArrayBuffer(hash);