fix bug where you can't use isForMe without payloadKeypair

It should be possible to check to see if a message isForMe with only the
scanKeypair, and not the payloadKeypair. There was a bug where only the
scanKeypair was being used to produce the receiveKeypair, but this was a
mistake. Both the scanPubkey and payloadPubkey should be necessary to produce
the receivePubkey, and both the scanPrivkey and payloadPrivkey should be
necessary to produce the receivePrivkey. If an online computer has only the
public keys of both (and the scanPrivkey), then that is good enough to check
for isForMe.