Fix errors

lib/repubmark/elems/footnote.rb

@@ -39,10 +39,10 @@ module Repubmark
           end
           result += %(<a href="#{url}">#{link_text}</a>)
         else
-          result += link_text
+          result += link_text.to_s
         end
         result += "\n"
-        result += @caption.to_html
+        result += @caption.to_html.to_s
         if alt_urls&.any?
           result += %[(#{
             alt_urls.map do |alt_url|