Bug#918659: Debootstrap started to fail with Busybox

[Piotr Jurkiewicz <piotr.jerzy.jurkiewicz@gmail.com>]

Package: debootstrap
Version: 1.0.113

Debootstrap started to fail with busybox since version 1.0.113. 
Previously it worked flawlessly with busybox. It fails silently, but 
debootstrap.log contains the following error:

    date: invalid date 'Tue, 15 Jan 2019 02:23:06 UTC'

The reason is commit: 75cd76ca821bfe92434b26503b1716ca6b99fd0c, which 
parses Valid-Until using `date -d` command. Busybox's date command does 
not parse RFC 2822 dates.

Please do not use `date -d` command for parsing Valid-Until field.

Or, at least, shortcut with NO_CHECK_VALID_UNTIL **before** `date -d` 
command is called (so one will use --no-check-valid-until on Busybox to 
prevent debootstrap from silent fail).