The behavior is now consistent with ReflectionMethod.

Instead specify a default value for the argument, which is more
idiomatic.

PHP does not allow static calls to non-static internal methods
anyway, so these checks are redundant.

And cleanup the implementation to perform a normal by-value
assignment.

For symmetry with zend_read_property_ex.

This was broken by 6dc0cd868d972885249ce40f2d64097cd0ba6c1f, which
moved static property initialization outside of constant updating.

Instead of replicating logic, use zend_get_static_property() API
in the reflection implementation, just like we're using
read_property for non-static proprety access.