[5.6] Use input method to allow compatibility with Lumen and PusherBroadcaster

[davidstanley01]

When using Lumen, the route resolver as called in \Illuminate\Http\Request::route will generally return an array and not a \Illuminate\Routing\Route object. This presents a problem whenever the __get magic method on the Request object is called.

In \Illuminate\Broadcasting\Broadcasters\PusherBroadcaster::decodePusherResponse, the first conditional uses that magic method to check if a callback is present. This change was introduced in #24018 and is the root cause of this issue.

To resolve this issue, we should use the input method to check for a callback rather than the current style. The change introduced here does just that and will fix laravel/lumen-framework#763.

[deleugpn]

Would this be a good candidate for tests to avoid regression?