Fix reflection creation
Fix reflection creation

* I didn't write this: just applied from @hgillane's fork
* Support two-part reflection creation process: AR::Reflection.create, add_reflection
* Previous direct-creation path is still there

https://github.com/hgillane/active_hash/commit/51a47558a0ee1813d4818f49e2688dc85851d46d

a2ikm/active_hash,kbrock/active_hash,zilkey/active_hash