Fixes issue #834

Rewrite wait_for_upload_id function so UploadCancelledError is raised
when state is _CANCELLED, previous logic didn't raise UploadCancelledError
if upload_id was set.
Added test_cancel_after_upload_id to test_tasks

This test creates an upload_task, sets the upload_id, and then cancels the upload.

At this point any call to wait_for_upload_id must raise an UploadCancelledError,
even if a valid upload_id is set on the MultipartUploadContext object.
Merge branch 'jlopex-cancel_pending_tasks' into develop

* jlopex-cancel_pending_tasks:
  Added test_cancel_after_upload_id to test_tasks
  Fixes issue #834
Add changelog entry for #834