Use Jekyll::Posts for both LSI indexing and lookup.

When looking for related posts, Jekyll was indexing `Jekyll::Post`
objects, but finding related posts based on `Jekyll::Post#content`. This
caused two problems:

1. Ruby 2.2 will warn on == if <=> throws an exception (and future Ruby
versions will surface that exception). Because `String`s can't be
compared with `Jekyll::Post`s, this warning was appearing all the time
while searching for related posts.

2. LSI won't return a post itself when searching for related posts. But
LSI could never tell that we were searching on a post, since Jekyll
passed post content, not a post object. With this fix, we can remove the
`- [post]` from `Jekyll::RelatedPosts#find_related`.

This is a more accurate fix for #3484.

lib/jekyll/related_posts.rb

@@ -42,7 +42,7 @@ module Jekyll
     end
 
     def lsi_related_posts
-      self.class.lsi.find_related(post.content, 11) - [post]
+      self.class.lsi.find_related(post, 11)
     end
 
     def most_recent_posts