issue-10750: Fix ISO8601 i18n formatting

When a Time or a Date is created using an ISO8601 UTC timezone,
attempting to execute i18nFormat() on it results in an error and
the timestamp not being formatted. This patch fixes that and adds
the requisite tests.

src/I18n/DateFormatTrait.php

@@ -209,7 +209,7 @@ trait DateFormatTrait
         $key = "{$locale}.{$dateFormat}.{$timeFormat}.{$timezone}.{$calendar}.{$pattern}";
 
         if (!isset(static::$_formatters[$key])) {
-            if ($timezone === '+00:00') {
+            if ($timezone === '+00:00' || $timezone === 'Z') {
                 $timezone = 'UTC';
             } elseif ($timezone[0] === '+' || $timezone[0] === '-') {
                 $timezone = 'GMT' . $timezone;

tests/TestCase/I18n/DateTest.php

@@ -113,6 +113,11 @@ class DateTest extends TestCase
 
         $result = $time->i18nFormat(\IntlDateFormatter::FULL, null, 'es-ES');
         $this->assertContains('14 de enero de 2010', $result, 'Default locale should not be used');
+
+        $time = new $class('2014-01-01T00:00:00Z');
+        $result = $time->i18nFormat(\IntlDateFormatter::FULL, null, 'en-US');
+        $expected = 'Wednesday, January 1, 2014 at 12:00:00 AM GMT';
+        $this->assertEquals($expected, $result);
     }
 
     /**

tests/TestCase/I18n/TimeTest.php

@@ -506,6 +506,11 @@ class TimeTest extends TestCase
         $result = $time->i18nFormat(\IntlDateFormatter::FULL);
         $expected = 'Wednesday January 1 2014 12:00:00 AM GMT-01:30';
         $this->assertTimeFormat($expected, $result);
+
+        $time = new $class('2014-01-01T00:00Z');
+        $result = $time->i18nFormat(\IntlDateFormatter::FULL);
+        $expected = 'Wednesday January 1 2014 12:00:00 AM GMT';
+        $this->assertTimeFormat($expected, $result);
     }
 
     /**