Fix image kind names

I accidentally broke these refactoring a .map() to a .transform_values()
even though I was using the key. Before this change .name() was the
class name (Whitehall::ImageKinds), not the name of the image kind.

lib/whitehall/image_kinds.rb

@@ -39,7 +39,7 @@ def version_names
 
   class ImageKinds
     def self.build_image_kinds(hash)
-      hash.transform_values { |config| ImageKind.new(name, config) }.freeze
+      hash.to_h { |name, config| [name, ImageKind.new(name, config)] }.freeze
     end
   end
 end

test/unit/lib/whitehall_test.rb

@@ -27,6 +27,7 @@ class WhitehallTest < ActiveSupport::TestCase
   test "Whitehall.image_kinds is populated with defaults from config" do
     assert_pattern do
       Whitehall.image_kinds["default"] => {
+        name: "default",
         valid_width: 960,
         valid_height: 640,
         versions: [