Description:
------------
Micro seconds on the date property (and consequently, formatted output) do not update after the initial creation of the DateTime object.

Test script:
---------------
$datetime = Datetime::createFromFormat('Y-m-d H:i:s.u', '2009-02-09 15:16:17.015750');
$datetime->setTimeStamp(0);
print_r($datetime);
	
$datetime = Datetime::createFromFormat('Y-m-d H:i:s', '2009-02-09 15:16:17');
$datetime->setTimeStamp(0.123);
print_r($datetime);

Expected result:
----------------
DateTime Object
(
    [date] => 1970-01-01 01:00:00.000000
    [timezone_type] => 3
    [timezone] => Europe/Berlin
)
DateTime Object
(
    [date] => 1970-01-01 01:00:00.123000
    [timezone_type] => 3
    [timezone] => Europe/Berlin
)

Actual result:
--------------
DateTime Object
(
    [date] => 1970-01-01 01:00:00.015750
    [timezone_type] => 3
    [timezone] => Europe/Berlin
)
DateTime Object
(
    [date] => 1970-01-01 01:00:00.000000
    [timezone_type] => 3
    [timezone] => Europe/Berlin
)