Solve incompatibility with autoload

Fix for a small compatibility problem with the autoload plugin of Daniel Ennis (https://github.com/aikar/autoloader)

In lib/packets/execute.js at line 33, the code:

        if (toString.call(this.parameters[i]) == '[object Date]') {

confuses autoloader, because the context is missing.

I changed it to:

         if (Object.prototype.toString.call(this.parameters[i]) == '[object Date]') {