Ref #4802: HTTP_Header not compatible with ArrayObject

HHVM complains about this. The Zend engine silently ignore the bug, probably casting the NULL internally to int 0.

Could not create a test for it, as this works with Zend, but it's fatal in HHVM.
Merge pull request #424 from enov/patch-3

Ref #4802: HTTP_Header not compatible with ArrayObject