Improved the Zend compatibility for ReflectionClass

Summary: `inNamespace()`, `getNamespaceName()`, and `getShortName()` should not be `final`. This was causing issues in phpspec which mocks these methods.

This was done for ReflectionFunctionAbstract in #3070 but ReflectionClass was missed

Closes https://github.com/facebook/hhvm/pull/3233

Reviewed By: @ptarjan

Differential Revision: D1445618