signMessage API returns wrong type according to SEP43

[aristidesstaffieri]

SEP43 defines the response for signMessage to be

Promise<{
      signedMessage: string;
      signerAddress: string;
    } & error?: Error>,

but our response today is

Promise<{
      signedMessage: Buffer like object;
      signerAddress: string;
    } & error?: Error>,

https://github.com/stellar/stellar-protocol/blob/master/ecosystem/sep-0043.md

This was originally reported by Enrique here #1626