Follow the EVENT_TYPE_CHAIN contract

As per the documentation, the EVENT_TYPE_CHAIN should always return the first set of parameters (http://www.mantisbt.org/docs/master/en/developers/dev.events.types.html#DEV.EVENTS.TYPES.CHAIN).

In this current implementation, the event EVENT_UPDATE_BUG is not usable by any other plugin once ProjectManagement has used it.

This is because the chaining of the first set of parameters is not done by the update_bug function, which corrupts the value of those parameters for any following hooked functions.