Fix content_type property

The property "Content-type" does not exist. Use "content_type" instead.

Work for RabbitMQ.

src/Monolog/Handler/AmqpHandler.php

@@ -65,7 +65,7 @@ class AmqpHandler extends AbstractProcessingHandler
                 0,
                 array(
                     'delivery_mode' => 2,
-                    'Content-type' => 'application/json',
+                    'content_type' => 'application/json',
                 )
             );
         } else {