Fix verifyPresentation (#113)

The "verifyPresentation()" method was previously checking if a stream
had been revoked. However, when dealing with stream updates, the stream
remains unrevoked, causing "verifyPresentation()" to incorrectly return
true for old documents as well. This problem has been resolved by 
implementing the new check.

demo/src/utils/verifyPresentation.ts

@@ -29,6 +29,9 @@ export async function verifyPresentation(
  if (stream.revoked) {
  return false
  }
+ if (stream.streamHash !== presentation.documentHash) {
+ return false
+ }
  return trustedIssuerUris.includes(stream.issuer)
  } catch {
  return false