Optimize Msg decode

[logan059]

Before msg.to_hex_data() is return b'd601233...' ，but now return 'd601233...'

[mchardysam]

@logan059 you're right that the docstring says hex string and it hands back bytes, so the two don't match.

I haven't taken it as is though, because test_messages.py asserts the expected values as bytes and anyone already calling to_hex_data() will be handling bytes today, so it'd break them without warning. I'd rather do it deliberately with the tests updated and a note in the changelog.

Leaving it open as the thing to fix. Are you decoding it yourself at the call site at the moment, or would a separate method that returns str be more useful to you?