Fix directory resource output and exists check (#1950)

* Fix to_s on directory resource

The `to_s` method on the `directory` resource is not defined
in the correct class, leading `directory` resources to be printed
as the parent resource (`file`) instead.

Signed-off-by: Adam Leff <adam@leff.co>

* Directory existence should check to see if it's a directory

Signed-off-by: Adam Leff <adam@leff.co>

lib/resources/directory.rb

@@ -13,9 +13,13 @@ module Inspec::Resources
         it { should be_directory }
       end
     "
-  end
 
-  def to_s
-    "Directory #{@path}"
+    def exist?
+      file.exist? && file.directory?
+    end
+
+    def to_s
+      "Directory #{source_path}"
+    end
   end
 end