Set default payload

Land #6849 for this to be effective.

modules/exploits/unix/fileformat/imagemagick_delegate.rb

@@ -55,6 +55,7 @@ class MetasploitModule < Msf::Exploit
       ],
       'DefaultTarget'   => 0,
       'DefaultOptions'  => {
+        'PAYLOAD'               => 'cmd/unix/reverse_netcat',
         'DisablePayloadHandler' => false,
         'WfsDelay'              => 9001
       }