Juju set should give feeback that the value is already set

[Matthew Williams]

juju set mycharm foobar=12345 gives no feedback if foobar=12345.

During charm debugging it is common to be logged into the unit tailing the charm log watching for changes. If juju set is called but the charm already has that config value set then nothing will appear in the log. Everything is working correctly as juju doesn't need to set anything on the unit. But it would be useful for the client to respond that nothing has been set because the value hasn't changed