system-source: Allow messages to flow through JSON

In case a message does not match the JSON marker, allow them to flow
through unmodified. This makes system() work with non-CIM messages
again.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
Merge pull request #245 from algernon/f/3.6/system-source/json-channels

system-source: Allow messages to flow through JSON