Description:
------------
The \IntlDateFormatter constructor returns 'null' when provided a 'GMT+00:00' timezone.
The correct bevahior would be to return an instance of \IntlDateFormatter.

My guess is that the problem lies in intl-3.0.0/timezone/timezone_class.cpp at line 202-211:

if (timeZone->getID(gottenId) != id) {
    [...]

    return NULL;
}

The problem is that, in the specific case of 'GMT+00:00', the 'timeZone->getID(gottenId)' evaluates to 'GMT', thus the inequality is sastified.

(in PHP: IntlTimeZone::createTimeZone('GMT+0N:00')->getID() is 'GMT+0N:00' if N is not 0, and 'GMT' if N = 0)

Test script:
---------------
$dt = [new \DateTime('2015-01-01 00:00:00 +00:00'), new \DateTime('2015-01-01 00:00:00 +01:00')];
$tz = ['GMT+00:00', 'GMT+01:00'];

foreach (array_merge($dt, $tz) as $timezone) {
    if ($timezone instanceof \DateTime) {
        $timezone = $timezone->getTimeZone();
    }

    $formatter = new \IntlDateFormatter(
        'fr_FR',
        \IntlDateFormatter::NONE,
        \IntlDateFormatter::NONE,
        $timezone
    );

    if (is_null($formatter)) {
        var_dump($timezone);
    }
}

Expected result:
----------------
Nothing, as the $formatter should never be 'null'.

Actual result:
--------------
object(DateTimeZone)#3 (2) {
  ["timezone_type"]=>
  int(1)
  ["timezone"]=>
  string(6) "+00:00"
}

string(9) "GMT+00:00"