Fix usage of Protobuf.js Message type in TS file

[murgatroid99]

This should fix #176.

[kjin]

nit -- If the payload has to be an object should it be Message<{}> instead?

[murgatroid99]

The Message type here is a Protobuf.js type, not a general type for any kind of message that might be transmitted by gRPC, so it's already an object. And honestly, I put any there because I have no idea what that type parameter means, even after reading Protobuf.js's .d.ts file.