Add test for #+description keyword

test/site/content-org/single-posts/post-toml.org

@@ -7,6 +7,7 @@
 #+hugo_tags: single toml "cross-link"
 #+hugo_categories: cat1 cat2
 #+hugo_menu: :menu "foo" :weight 10 :parent main :identifier single-toml
+#+description: Some description for this post.
 
 This is a single post. You do not need to set the =EXPORT_FILE_NAME=
 property in here. But then you also lose the tag and property

test/site/content/singles/post-toml.md

@@ -1,5 +1,6 @@
 +++
 title = "Single Post with TOML front matter"
+description = "Some description for this post."
 date = 2017-07-20
 tags = ["single", "toml", "cross-link"]
 categories = ["cat1", "cat2"]