[stable6] Upload abortion is now detected within the  OC_Connector_Sabre_File::put...

...()

OC_Connector_Sabre_AbortedUploadDetectionPlugin is pointless

Adding unit test testUploadAbort()

Backport of https://github.com/owncloud/core/pull/9507 to stable6

Please review @DeepDiver1975 and check whether I fixed the conflicts properly.
I had a quick test and aborting worked fine.