Ask about 'Draft' before asking about 'Post'

`Draft`s are a subclass of `Post` so `draft.is_a?(Post)` will return
`true`, thus making all `Draft`s `Post`s, which is not desired. If
asking about `Draft` first, then we avoid this problem.

Fixes #2726

lib/jekyll/convertible.rb

@@ -128,12 +128,12 @@ module Jekyll
     #
     # Returns the type of self.
     def type
-      if is_a?(Post)
+      if is_a?(Draft)
+        :drafts
+      elsif is_a?(Post)
         :posts
       elsif is_a?(Page)
         :pages
-      elsif is_a?(Draft)
-        :drafts
       end
     end