Added ability to add message_properties with variables.

When adding message_properties dictionary (message_properties => {"content_type" => "const type"})
You can now use dynamic values from variables {"content_type" => "%{ContentType}"}

lib/logstash/outputs/rabbitmq.rb

@@ -51,8 +51,8 @@ def register
         @thread_local_exchange = java.lang.ThreadLocal.new
       end
 
-      def symbolize(myhash)
-        Hash[myhash.map{|(k,v)| [k.to_sym,v]}]
+      def symbolize(myhash, event)
+        Hash[myhash.map{|(k,v)| [k.to_sym, if v.is_a? String event.sprintf(v) else v end]}]
       end
 
       def multi_receive_encoded(events_and_data)
@@ -63,7 +63,7 @@ def multi_receive_encoded(events_and_data)
 
       def publish(event, message)
         raise ArgumentError, "No exchange set in HareInfo!!!" unless @hare_info.exchange
-        local_exchange.publish(message, :routing_key => event.sprintf(@key), :properties => symbolize(@message_properties.merge(:persistent => @persistent)))
+        local_exchange.publish(message, :routing_key => event.sprintf(@key), :properties => symbolize(@message_properties.merge(:persistent => @persistent), event))
       rescue MarchHare::Exception, IOError, AlreadyClosedException, TimeoutException => e
         @logger.error("Error while publishing. Will retry.",
                       :message => e.message,
@@ -97,4 +97,4 @@ def close
       end
     end
   end
-end
+end