Add microseconds to the serialised form of DateTime objects.

Fixes bug #67308 (Serialize of DateTime truncates fractions of second).
Merge branch 'PHP-5.4' into PHP-5.5

* PHP-5.4:
  Add microseconds to the serialised form of DateTime objects.
Merge branch 'PHP-5.5' into PHP-5.6

* PHP-5.5:
  Add microseconds to the serialised form of DateTime objects.