Add coverage for tcp input event decoration w/ 'host' field
Only set 'host' field if it's not already set in the event.

Ditto for 'sslsubject'

This fixes a bug where tcp w/ json_lines codec would overwrite any
'host' setting with the client address on the other side of the
connection.
Merge pull request #937 from jordansissel/fix-tcp-input-event-decoration

Fix tcp input event decoration
Merge git://github.com/logstash/logstash