Simplified Attribute.determine_type case statement with Class#singleton_class
Simplified Attribute.determine_type case statement with Class#singleton_class

* This method is part of the 1.9 core API, and is only available in 1.8
  because backports is a development dependency.

nuclearsandwich/virtus,hapiben/virtus,kristianmandrup/virtus,artemeff/virtus,macressler/virtus,solnic/virtus,reevoo/virtus,mallikarjunayaddala/virtus,dslh/virtus,faucct/virtus,baojjeu/virtus,XescuGC/virtus,lgierth/virtus,JustMikey/virtus