File.dirname: add spec for removal of repeated leading slashes

I discovered that surprising behavior while trying to optimize
`File.dirname`.

Author: byroot

core/file/dirname_spec.rb

@@ -79,6 +79,10 @@ def object.to_int; 2; end
   end
 
   platform_is_not :windows do
+    it "ignores repeated leading / (edge cases on non-windows)" do
+      File.dirname("/////foo/bar/").should == "/foo"
+    end
+
     it "returns all the components of filename except the last one (edge cases on non-windows)" do
       File.dirname('/////').should == '/'
       File.dirname("//foo//").should == "/"