fix(drafts): avoid error due to missing dates(#178)

Resolves #177

Co-Authored-By: leviem1 <leviem1@users.noreply.github.com>

Author: Haroenv

lib/jekyll/algolia/overwrites/jekyll-document.rb

@@ -5,9 +5,10 @@ module Jekyll
   class Document
     # By default, Jekyll will set the current date (time of build) to any
     # collection item. This will break our diff algorithm, so we monkey patch
-    # this call to return nil if no date is defined instead.
+    # this call to return nil if no date is defined and the file is not a
+    # draft instead.
     def date
-      data['date'] || nil
+      data["date"] ||= (draft? ? source_file_mtime : nil)
     end
   end
 end