Fix #3117: Nicoseiga handler grabbing wrong commentary source

* `summary` is the wrong field. It's the list of comments left by users,
  not the artist's commentary.

* For some reason `doc.response.image.description` returns nil even
  though the description element exists. Switch to `Hash.from_xml` to
  avoid this.

test/unit/sources/nico_seiga_test.rb

@@ -23,7 +23,7 @@ module Sources
 
       should "get the artist commentary" do
         assert_equal("コジコジ", @site_2.artist_commentary_title)
-        assert_equal("懐かしいですよね。テ また懐かしいものを ", @site_2.artist_commentary_desc)
+        assert_equal("コジコジのドット絵\nこんなかわいらしい容姿で毒を吐くコジコジが堪らん（切実）", @site_2.artist_commentary_desc)
       end
 
       should "get the image url" do