[intl] IntlDateFormatter::format polyfill does not support \DateTimeInterface

[Tobion]

IntlDateFormatter::format supports DateTimeInterface since php 7.1.5, see https://www.php.net/manual/en/intldateformatter.format.php

7.1.5 | Support for providing general DateTimeInterface objects to the value parameter was added. Formerly, only properDateTime objects were supported.

But symfony/intl polyfill does not, see

symfony/src/Symfony/Component/Intl/DateFormatter/IntlDateFormatter.php

Lines 198 to 200 in 9857e4d

	if (!\is_int($timestamp) && !$timestamp instanceof \DateTime) {
	$argumentError = sprintf('datefmt_format: string \'%s\' is not numeric, which would be required for it to be a valid date', $timestamp);
	}