Fix empty hash fixture not returning self

The return value of each would be the same as the expected value for
calls to `#first`, `#last`, etc.

Author: elia

core/enumerable/fixtures/classes.rb

@@ -38,12 +38,14 @@ def each(*arg)
   class Empty
     include Enumerable
     def each
+      self
     end
   end
 
   class EmptyWithSize
     include Enumerable
     def each
+      self
     end
     def size
       0