mtime ignored when copying files to Nextcloud via davfs

[boycott-github]

Steps to reproduce:

• create user test password test on a Nextcloud 13.0.5 instance at https://example.com
• Provision a Ubuntu 18.04 from scratch
• sudo apt-get update
• sudo apt-get install davfs
• echo 'https://example.com/remote.php/dav/files/test test test' >> /etc/davfs2/secrets
• mount -t davfs https://example.com/remote.php/dav/files/test /mnt
• ls -l /etc/timezone
  -rw-r--r-- 1 root root 8 Apr 26 05:10 /etc/timezone
• cp -a /etc/timezone /mnt/
• ls -l /mnt/timezone
  -rw-r--r-- 1 root root 8 Apr 26 05:10 /mnt/timezone
• umount /mnt
• mount -t davfs https://example.com/remote.php/dav/files/test /mnt
• ls -l /mnt/timezone
  -rw-r--r-- 1 root root 8 Aug 27 15:48 /mnt/timezone

And checking the time via the Nextcloud web interface confirms the mtime is not what it should be.

[nextcloud-bot]

GitMate.io thinks possibly related issues are #6911 (Is there any file chooser for nextcloud), #9910 (Nextcloud locking .PPTX files), #6223 (Unable to upload files to NextCloud), #6316 (Managing long file names (nextcloud)), and #5438 (Nextcloud won't upload larger files).

[MorrisJobke]

Duplicate of #3572

Currently we only store the upload time if a specific header is not set explicitly. So first we need a way to store it and then we can support the mtime and ctime properly.