Better distinguish between post and category

The constructor currently forgets the difference between a single post
and a category with only one post in it by wrapping any single post in
an array.  This causes the category to get the same generated URL as
the post itself, instead of using the configured bundle permalink
template.

To fix this, this patch teaches the constructor to remember whether it
received only a single post or an array containing a single post in
it.

lib/jekyll-pandoc-multiple-formats/pandoc_file.rb

@@ -26,7 +26,7 @@ module Jekyll
     include Convertible
 
 
-    attr_reader :format, :site, :config, :flags, :posts, :slug, :title, :url
+    attr_reader :format, :site, :config, :flags, :posts, :slug, :title, :url, :is_single_post
     attr_reader :papersize, :sheetsize, :signature
 
     def initialize(site, format, posts, title = nil)
@@ -34,16 +34,17 @@ module Jekyll
       @config = JekyllPandocMultipleFormats::Config.new(@site.config['pandoc']).config
       @format = format
       @flags  = []
+      @is_single_post = not(posts.is_a? Array)
 
-      if posts.is_a? Array
+      if single_post?
+        @posts = [posts]
+        @title = title or posts.data['title']
+      else
         @posts = posts
 
         raise ArgumentError.new "'title' argument is required for multipost file" unless title
 
         @title = title
-      else
-        @posts = [posts]
-        @title = title or posts.data['title']
       end
 
       @slug = Utils.slugify(@title)
@@ -236,7 +237,7 @@ module Jekyll
     end
 
     def single_post?
-      @posts.count == 1
+      @is_single_post
     end
 
     def has_cover?