Msgpack bytestring/text encoding

### Subject of the issue

Hi! I attached msgpack encoding of block `9414973` from betanet (retrieved via [url](https://api.betanet.algoexplorer.io/v2/blocks/9414973?format=msgpack)). It doesn't meet [msgpack specification](https://github.com/msgpack/msgpack/blob/master/spec.md#str-format-family).

It encodes some address field of a transaction as string. However specification of messagepack clearly says that this encoding is for UTF-8 strings, not bytestring.

### Attachments

[file.zip](https://github.com/algorand/go-algorand/files/6135149/file.zip)

### Your environment

N/A

### Steps to reproduce

Try parsing attached file with some msgpack parser (e.g. this Haskell [library](https://github.com/TokTok/hs-msgpack-binary)).

### Expected behaviour

File is parsed.

### Actual behaviour

File is not parsed due to wrongly encoded bytestring field.