fix deserialization of schema'd mummy messages

looks like there was an error in datahog's consumption of mummy? i'm not 100% clear on why it was wrong, but i discovered something that worked by poking at mummy in the interpreter. 

if you want some more details on the error, here's [a repro script + output](https://gist.github.com/cameron/11756d03053f72a334b6)