If a existing file in Shared/ with update permissions gets updated we need to write the .part file to a different place because we can't create new files in the Shared folder
updating a existing large file creates new file chunks. Therefore createFile() needs to check not only if we can write to the parent folder but also if we can update the existing file"
part file needs to have .part extension
Merge pull request #5193 from owncloud/fix_5126_2

fix upload to /Shared