SPL: Fix for bug 66405 RecursiveDirectoryIterator with CURRENT_AS_PATHNAME

Currently when you use RecursiveDirectoryIterator with the
CURRENT_AS_PATHNAME flag PHP will throw an UnexpectedValueException with
message 'Objects returned by RecursiveIterator::getChildren() must implement
RecursiveIterator'.This happend because getChildren() will return the
current directory name instead of an Iterator (or subclass of) as required
by the RecursiveIterator interface.

This commit changes getChildren() to return annother
RecursiveDirectoryIterator but current() still returns the path name when
CURRENT_AS_PATHNAME is used. A PHPT test case (bug66405.phpt) that
reproduces the bug is included.

This fix was originally against the PHP-5.4 branch, but it was not merged
before 5.4 reached EOL. I am hoping to get it applied to 5.5, 5.6, and
master.
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