Serialized @Background task cancellation bugfix

If a serialized @Background task was cancelled after it had been
submitted to the executor but before its run() method was called, then
the following tasks with the same serial identifier were not executed.

Issue reported here:
https://github.com/excilys/androidannotations/pull/579#issuecomment-19173978

Detected by ThreadActivityTest#cancellableSerializedBackgroundTasks()
(but not always due to the race condition)
Merge pull request #624 from rom1v/cancel_background_fix

Serialized @Background task cancellation bugfix