Unfortunately multi part upload can't upload zero length files so
bring back the single part upload for zero length files only.
This was broken when we made all uploads multipart uploads.
Because of a bug in the Onedrive API it will sometime report the wrong
size. If the size is wrong other remotes that depend on the size might
fail. To fix this we overwrite the objects size with the real size
from ContentLength header.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
