Patch lumberjack protocol to work around expected event structure from t...

...he standalone forwarder

The standalone forwarder sends the event with the single key field ``line``.  If the input is instead part of a forwarder chain from logstash to logstash, then the original logic would eat the event entirely.  The assumption made here is that the chained event contains a ```message``` key if it is from another logstash instance, and does not if it is from the standalone forwarder.

Based on that check, the revised logic either accepts the event as is, or invokes the original logic on the ```line``` field.