Serialized DateTime includes fractions of seconds since 5.6.0beta4

https://github.com/php/php-src/blob/php-5.6.0beta4/NEWS
> Date: Fixed bug #67308 (Serialize of DateTime truncates fractions of second). (Adam)


```
1) Zend_Ldap_ConverterTest::testToLdapSerialize with data set #2 ('O:8:"DateTime":3:{s:4:"date";s:19:"1970-01-01 00:00:00";s:13:"timezone_type";i:1;s:8:"timezone";s:6:"+00:00";}', DateTime)
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'O:8:"DateTime":3:{s:4:"date";s:19:"1970-01-01 00:00:00";s:13:"timezone_type";i:1;s:8:"timezone";s:6:"+00:00";}'
+'O:8:"DateTime":3:{s:4:"date";s:26:"1970-01-01 00:00:00.000000";s:13:"timezone_type";i:1;s:8:"timezone";s:6:"+00:00";}'
```