bug #1533 Make date_modify work with DateTimeImmutable (globin)

This PR was squashed before being merged into the 1.16-dev branch (closes #1533).

Discussion
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Make date_modify work with DateTimeImmutable

Use the return value of `$date->modify` because `DateTimeImmutable::modify`
returns the modified object and does not modify the object modify was
called on.

Commits
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a5fe46f Make date_modify work with DateTimeImmutable