The clock class should only return instances of DateTimeInterface

[thkoch2001]

Although DateTimeInterface is only available from PHP 5.5 we should already declare the return values as such:
http://php.net/manual/de/class.datetimeinterface.php

All methods that now return a DateTime instance should instead call a method that prepares the return value:

• If the given value already is an instance of DateTimeImmutable than return it.
• Otherwise return a copy of the given DateTime instance

This is necessary to avoid other code to mess up our internal DateTime instance.