Fixes decoding emoji slates that contain a slate with non-ASCII characters

[NicolasFlamel1]

Fixes decoding emoji slates that contain messages with non-ASCII characters.

This can be tested by running the following commands
epic-wallet send -m emoji -g "😃" 1
epic-wallet receive -m emoji -i "..."

Without this PR you'll get the following errors since participant messages containing non-ASCII characters, like 😃, aren't decoded correctly.

ERROR verify_messages - participant message doesn't match signature. Message: "ð"
ERROR Error validating participant messages: Signature error: Optional participant messages do not match signatures
Wallet command failed: LibWallet Error: Signature error: Optional participant messages do not match signatures

[who-biz]

Thanks Nicolas! Will review ASAP. Emoji transactions were written by the Brick Abode team, who are now taking a less active role in maintaining our repositories/writing code.

This feature was added right around the time I came onboard to the project, so may take me a bit to review.

Thanks for the first PR(s)!

[who-biz]

@johanneshahn have you tested and confirmed this issue, and that it resolves the error?

[who-biz]

Adding a note here to say that I'll test this myself soon, and we'll get it merged. Looks fine to me.