Fix issue where activity is null

[rojtjo]

Currently when using the facade it always returns the same PendingActivityLog object because facade instances are cached by default. This cuases the following issues:

• The $activity property is set to null after calling the log method.
  This causes an issue when starting the chain using the event() or causedByAnonymous() methods as they expect the property to be set
• The $log_name property is not set on the activity for consecutive calls

While disabling the caching of the facade already fixes both of these issues, I also included df95425 to ensure the activity is always accessed the same way.

An alternative fix would be to always return a new instance of ActivityLogger from the PendingActivityLog class.

Let me know if you prefer the alternative fix instead!