Description:
------------
Using array_walk() on parameters prior to sending them to MessageFormatter::format breaks date parameters and causes an error. Doesn't matter if you pass a \DateTimeInterface or an integer timestamp.

Test script:
---------------
$today = new DateTime();
$formatter = new \MessageFormatter('en_US', 'Today is {today, date, full}.');
$params = ['today' => $today];
array_walk($params, fn() => 1);
var_dump($params);
if($str = $formatter->format($params)) {
    echo $str;
} else {
    echo sprintf('Unable to format message (error #%s): ', $formatter->getErrorCode()) . $formatter->getErrorMessage();
}

Expected result:
----------------
array(1) {
  ["today"]=>
  object(DateTime)#12 (3) {
    ["date"]=>
    string(26) "2021-02-17 14:10:54.229580"
    ["timezone_type"]=>
    int(3)
    ["timezone"]=>
    string(13) "Europe/Moscow"
  }
}
Today is Wednesday, February 17, 2021.

Actual result:
--------------
array(1) {
  ["today"]=>
  object(DateTime)#12 (3) {
    ["date"]=>
    string(26) "2021-02-17 14:10:54.229580"
    ["timezone_type"]=>
    int(3)
    ["timezone"]=>
    string(13) "Europe/Moscow"
  }
}
Unable to format message (error #1): The argument for key 'today' cannot be used as a date or time: U_ILLEGAL_ARGUMENT_ERROR