Change Attribute#instance_variable_name to be a Symbol
Change Attribute#instance_variable_name to be a Symbol

* The methods that instance_variable_name is passed to internally all coerce the
  String into a Symbol before performing the lookup, so this is a minor
  simplification.

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