Fix bugs related to shell requests

[mchitre]

Overview

This PR fixes 2 problems:

1. ShellExecReq property name should be command and not cmd. The latter worked because of the way GSON worked in Java, but now we have fixed the naming in Java and so the corresponding Julia message needed update.
2. The iterator on a GenericMessage had a bug, causing printing of GenericMessages to throw exceptions. We now have a better iterator.

What was broken

using Fjage

gw = Gateway("localhost", 1100)   # with fjage master running on localhost:1100
sh = agent(gw, "shell")

This was failing with a REFUSE:

sh << ShellExecReq(cmd="ps")

and this was not allowed:

sh << ShellExecReq(command="ps")

With cmd replaced by command in the message definition, this worked:

sh << ShellExecReq(command="ps")

but this was failing with an exception:

sh << ShellExecReq(command="ps", ans=true)

All of this works after this PR!

[ymtoo]

LGTM!