Update session.shell_upgrade to match rapid7#3401, lands rapid7#4058

Author: HD Moore

lib/msf/core/rpc/v10/rpc_session.rb

@@ -73,7 +73,7 @@ def rpc_shell_upgrade( sid, lhost, lport)
     s = _valid_session(sid,"shell")
     s.exploit_datastore['LHOST'] = lhost
     s.exploit_datastore['LPORT'] = lport
-    s.execute_script('spawn_meterpreter', nil)
+    s.execute_script('post/multi/manage/shell_to_meterpreter')
     { "result" => "success" }
   end