customtype of type struct cannot jsonpb unmarshal

[ericlagergren]

(Posting b/c you asked for a separate issue.)

See: #199 (comment)

[awalterschulze]

Would you like to attempt a fix?

[ericlagergren]

See my comment on the other issue. This one is much simpler so I think it's manageable :)

[awalterschulze]

Fixed by #239