Change date's default json encode format to valid ISO8601.

src/I18n/DateFormatTrait.php

@@ -54,7 +54,7 @@ trait DateFormatTrait
      * @var string|array|int
      * @see \Cake\I18n\Time::i18nFormat()
      */
-    protected static $_jsonEncodeFormat = "yyyy-MM-dd'T'HH:mm:ssZ";
+    protected static $_jsonEncodeFormat = "yyyy-MM-dd'T'HH:mm:ssxxx";
 
     /**
      * Caches whether or not this class is a subclass of a Date or MutableDate

tests/TestCase/I18n/DateTest.php

@@ -151,7 +151,7 @@ class DateTest extends TestCase
     public function testJsonSerialize($class)
     {
         $date = new $class('2015-11-06 11:32:45');
-        $this->assertEquals('"2015-11-06T00:00:00+0000"', json_encode($date));
+        $this->assertEquals('"2015-11-06T00:00:00+00:00"', json_encode($date));
     }
 
     /**

tests/TestCase/I18n/TimeTest.php

@@ -711,7 +711,7 @@ class TimeTest extends TestCase
     public function testJsonEnconde($class)
     {
         $time = new $class('2014-04-20 10:10:10');
-        $this->assertEquals('"2014-04-20T10:10:10+0000"', json_encode($time));
+        $this->assertEquals('"2014-04-20T10:10:10+00:00"', json_encode($time));
 
         $class::setJsonEncodeFormat('yyyy-MM-dd HH:mm:ss');
         $this->assertEquals('"2014-04-20 10:10:10"', json_encode($time));