change is_object() to gettype() === 'object' in spl for incomplete_class

PHP accepts __php_incompatible_class in spl. But objects from that class fail is_object.

This puts symfony from 98.68 to 98.8 percent.

I don't like to do this...
Can we run perf on this? I don't know how bad it is.

Test Plan:
`slow/ext_spl*` `zend/good/spl`