Fixes codec issue with logstash 7.2.0 and above

The fix is originally authored by João Duarte
logstash-plugins#55

Signed-off-by: Tero Saarni <[email protected]>

Author: tsaarni

lib/logstash/outputs/syslog.rb

@@ -131,8 +131,8 @@ def register
     if ssl?
       @ssl_context = setup_ssl
     end
-    
-    if @codec.instance_of? LogStash::Codecs::Plain
+
+    if @codec.class.to_s == "LogStash::Codecs::Plain"
       if @codec.config["format"].nil?
         @codec = LogStash::Codecs::Plain.new({"format" => @message})
       end