Fix #864 sign_builder_transaction sometimes doesn't work

[pmconrad]

sign_transaction calls get_required_signatures to find out which signatures need to be added. get_required_signatures will not report signatures that are already present. Later, however, sign_transaction removes the signatures that were already there, and only adds those reported by get_required_signatures.

The fix is to clear all signatures before calling get_required_signatures.

Resolves #864

[abitmore]

Good catch.

Actually I think sign_transaction should not remove old signatures when adding new signatures, then it will be more useful for multi-signing. By the way, if modified expiration and / or TaPoS fileds, the old signatures will become invalid. So I think it's better to check those fields as well, and try to not change them. Or perhaps add another command for this feature.

[pmconrad]

Yes, that might be a useful use-case. We shouldn't change the existing API call for this though. Better add a new one.