Fix copying of DateInterval objects

[theofidry]

Closes #75, #76.

[theofidry]

Kinda weird, it passes when debugging step by step but not when running the test without breakpoints.

@mnapoli any idea?

[mnapoli]

Not really :/ Maybe when you are debugging the date changes (seconds are passing) and when not debugging the test runs fast enough? Or maybe you are touching some weird behavior of PHP's internals ;)

[theofidry]

Going back on my words: we're may be introducing a BC here but more likely fixing a broken case.

[theofidry]

@mnapoli as there may be BC I would like you to review that one if you don't mind :)

[mnapoli]

I'm reading the diff and the other tickets but I'm not sure I understand the original problem. If I understand correctly what happens here is:

• DateTimeInterval are handled with an exception (like the other DateTime classes) -> good to me
• subclasses of DateTimeInterval is treated differently: why is that?

[theofidry]

Actually as the type filter is registered, DateInterval are handled by DateIntervalFilter. The change applied L192 is useless and needs to be reverted.

There is an inconsistency here, for DateTime and DateTimeZone we do a shallow copy with clone, which let the user define his kind of custom cloning if he wants to if he is inheriting from DateTimeZone. However a clone doesn't work on <PHP7.1 on DateInterval...