Description:
------------
When storing '015700' microseconds in a Datetime object,
Datetime::format('u') returns '015699'. This behavior is critical when
I use optimistic locking.

----
$datetime = Datetime::createFromFormat('u', '015700');
$datetime->format('u'); // => '015699'
----

Datetime objects hold microseconds as "double" type in C.
'015700' is held as 0.015699999999999998679. Datetime::format('u')
calls floor() and it returns .015699.
(see https://github.com/php/php-src/blob/PHP-5.3.18/ext/date/php_date.c#L1098 )


Test script:
---------------
$datetime = Datetime::createFromFormat('u', '015700');
$datetime->format('u');


Expected result:
----------------
015700

Actual result:
--------------
015699