applyEdit TASK_STATUS_CHANGE: do not set workerId

[nicoloboschi]

JIRA: https://dev.majordodo.org/jira/browse/MAJ-97

Reporter: caio @CaioK

Signature of the StatusEdit initializer is:
{code}
public static final StatusEdit TASK_STATUS_CHANGE(long taskId, String workerId, int finalStatus, String result)
{code}

but in applyModifications workerId is not set.

We should add this set, maybe raising an exception if new workerId != null and is not equal to the current workerId (in fact, the only circumstance when this parameter could be useful is to reset task workerId to null, since the real set of workerId is done by ASSIGN_TASK_TO_WORKER).