RecursiveRegexIterator's children don't use their parents flags

I don't think this is the right behaviour, but it matches php...

Also apply patch to bug68128 (Array to String conversion in accept())

The slow test I added tests for the children's flags. PHP reports NULL for pregFlags whereas we report 0.
I reported this at https://bugs.php.net/bug.php?id=68175

Fixes #3909

Test Plan:
`slow/ext_spl*`
`zend/good/ext/spl/`