TaskCompletedEngineEvent should contain variables passed on completion

[jangalinski]

Reference: this is a re-phrasing of "bug" #754

In camunda we complete a task with variables (form-data, ...) using taskService.complete(taskId, variableMap).

However, the variables set on completion do not end up in the TaskCompletedEngineEvent.
In our customers project, we encountered a situation where we use the TaskCompletedEngineEvent to further propagate state changes in the system, and noticed that the variables we are interested in where not contained in the event.

Feature request: Include the variables in the Complete-Command and -Event.