rest: fix upload of 0 length files

Before this change if ContentLength was set in the options but 0 then
we would upload using chunked encoding.  Fix this to always upload
with a "Content-Length" header even if the size is 0.

Remove workarounds for this from b2 and onedrive backends.

This fixes the issue for the webdav backend described here:

https://forum.rclone.org/t/code-500-errors-with-webdav-nextcloud/8440/

backend/b2/b2.go

@@ -1500,11 +1500,6 @@ func (o *Object) Update(in io.Reader, src fs.ObjectInfo, options ...fs.OpenOptio
 		},
 		ContentLength: &size,
 	}
-	// for go1.8 (see release notes) we must nil the Body if we want a
-	// "Content-Length: 0" header which b2 requires for all files.
-	if size == 0 {
-		opts.Body = nil
-	}
 	var response api.FileInfo
 	// Don't retry, return a retry error instead
 	err = o.fs.pacer.CallNoRetry(func() (bool, error) {