Fix exception creation when Errno is subclassed

Author: bjfish

core/exception/errno_spec.rb

@@ -56,3 +56,12 @@
     end
   end
 end
+
+describe "Errno::ENOENT" do
+  it "lets subclasses inherit the default error message" do
+    c = Class.new(Errno::ENOENT)
+    raise c, "custom message"
+  rescue => e
+    e.message.should == "No such file or directory - custom message"
+  end
+end